I changed my embouchure twice in college, the first time right after my sophomore year and the second during my Doctoral studies. It is never easy to make a 
big change and I have done more than my fair share. This photo is my placement today. Note that it is somewhat more than 2/3 upper lip, it is not quite over the top fleshy mound due to my heavy upper lip, and also it is not centered over my natural lip opening. Is it wrong?
Conventional thinking
Of course the standard thinking is 2/3 upper lip, a point emphasized by Farkas in The Art of French Horn Playing. To review, this is his central point on the topic:
The placement of the mouthpiece which is vitally important is its up-and-down position on the lips—that is, the proportion of the lower lip to upper lip in the mouthpiece. For this placement there is no hard and fast rule and probably no two horn players have exactly the same setting. The locating of each individual player’s exact, ideal spot has tremendous bearing on his playing ability…. There is one general placement which most horn players agree on…. The setting of the mouthpiece on the lips seems to result in “2/3 and 1/3” remarkably often in all brass teaching. It appears to be necessary to use this approximate proportion of one lip to the other in the mouthpiece.
Farkas goes into the topic in a lot of depth in this volume and in The Art of Brass Playing, and pretty much every book after Farkas addresses the topic to some degree.
What about placement side to side?
I’ll just state this before going any further: I don’t think it matters much. Many players have their embouchure off center, and most likely off center to the left side of their face. I have never personally run into a situation where moving the placement to a more centered one gave really any benefit. Which is to say …
You are an individual, one size does not fit all
Reading resources on this topic can be helpful, but you have to be careful of dogma that may sound right on paper but in reality may not work for you. In my own case I really can’t quite set up my embouchure as Farkas describes in print, with my heavy upper lip. I think a lot of players struggle to try to make their embouchure look like his description — they would be better to focus on the results.
As a principal goal of this Hornmasters series is too look more deeply at other classic and more recent resources, what is their take on this dogma?
Practical approaches
Gunther Schuller in Horn Technique passes this topic quickly but with the goal of simply finding the most natural placement possible. “Take the mouthpiece and place the upper half of the rim on the upper lip so that the rim catches” on the fleshy mound of the upper lip. The uppermost area of the rim will be into the white above the lip “and will guarantee that the mouthpiece is more or less well centered.” He also notes, however that few players are exactly centered in their placement and that this, “while generally desirable,” is not a prerequisite to the formation of a good embouchure. He also notes that an embouchure where the rim of the mouthpiece sits outside the red of both lips is “problematic and unsuccessful in a majority of cases.”
Harry Berv in A Creative Approach to the French Horn merely notes that
Once the embouchure position is set, it is time for the crucial mouthpiece placement. A slightly off-center mouthpiece placement is not necessarily incorrect. The formation of the teeth has a great deal to do with this….
With the embouchure in a semi-puckered position, the mouthpiece is placed on the lips. The mouthpiece should be placed in the center of the lips with two-thirds of the mouthpiece on the upper lip and one-third on the lower lip.
Douglas Hill in Collected Thoughts on Teaching and Learning, Creativity, and Horn Performance recommends to
…position the rim of your mouthpiece to rest with two-thirds to three-fourths of its circle on the top lip. The exact proportions depend on the shape of your own particular lips and the exact formation of your teeth. Wiggle the mouthpiece around within those proportions and find the most comfortable position. Once you get somewhat comfortable, remember that feeling and memorize what it looks like through the consistent use of a mirror. With the mouthpiece ready near the lips, form and create that clear and open buzz at the aperture. Hold that feeling and replace the mouthpiece on the lips while allowing the texture of the lips to relax just enough to absorb the rim of the mouthpiece. The pressure of the mouthpiece against the lips should be only enough to seal the leakage of air through the sides of the mouth.
More detailed accounts
William C. Robinson begins his discussion of the embouchure in An Illustrated Advanced Method for French Horn Playing with advice on placing and establishing the embouchure. In terms of how to establish the correct positioning, Robinson explains that “The outside edge of the mouthpiece should be placed no lower than the line at the bottom edge of the soft inner part of the lower lip.” His description of how to find the most natural aperture is rather detailed (too detailed to quote in this post, with illustrations) and involves daily setup of the aperture as part of your warmup with a mouthpiece visualizer.
Farquharson Cousins in On Playing the Horn has several items to consider on positioning the mouthpiece. He starts with the idea that you first make sure you are forming a good lip aperture with this valuable nugget included in his longer description.
The aperture is made as if to sound the French “rue” rather than the English “feet”. Within this aperture as much of the rosy inner lip is revealed as the tautness of the surrounding muscles allows.
Cousins also points out something to watch for with young students, and a solution.
A common fault made by beginners is to place the mouthpiece on the lips and then, at the moment of starting the note, to ‘buckle up’ the surrounding muscles as though to seize the mouthpiece in case it should run away. This is a negation of the stretched membrane and the result is a strangled and halting sound…. A good cure is to form the correct embouchure and place the thick end of a pencil between the lips. Then, without using the tongue, ‘hoof’ it out as though it were and orange pip, at the same time withdrawing the pencil with the hand. This should be done several times while watching in a mirror to see that there is no alteration in the muscle set-up.
When satisfied with what you see, place the mouthpiece and blow as before, but this time sustain the note. If the muscles are now properly conditioned, there will be no ‘buckling’, though probably at the first touch of the mouthpiece the embouchure will still buckle to some extent. However, perseverance will guarantee that ‘buckling’ can be ironed out. Here our mirror is most useful.
The big picture
Most sources that get into details focus on their own approach. Sources that actually look at the “big picture” are rare. This is one of the reasons why we will conclude this post with several quotes related to this important topic from Thoughts on Playing the Horn Well by Frøydis Ree Wekre.
One aspect of this publication that makes it especially valuable is that she openly recognizes that there is not just one valid approach to mouthpiece placement. She notes that most teachers agree that the mouthpiece should be placed horizontally in the middle of the mouth and approximately 2/3 upper lip, which may vary up to ¾ upper lip and down to ½ upper lip. Turning to the lower lip specifically Wekre continues,
The first major disagreement in this regard is how the lip should be set under the mouthpiece as it is being place, and how the teeth (jaw) should be positioned….
Some teachers recommend that the part of the lower lip should be rolled in under the upper lip, thus becoming invisible in a “normal” playing position. Other teachers, myself included, recommend that the mouthpiece should be placed directly on the lower lip without altering the position of the lip from its natural position.
The rolled-in lower lip is problematic in her view because it creates a large break which impacts flexibility, the sound can be “less homogeneous and centered,” and it is difficult to manage soft dynamics. She then lays out a problem that she has seen in her teaching.
There is, in addition, a philosophy that the embouchure will “blow itself in,” and the end justifies the means, as long as it sounds good. This philosophy….. is based on the fact that many successful horn players have an unorthodox, self-taught or random playing technique. Their talent has permitted them to find a technique with which they can manage the music and one that works. This way of thinking is also due to a wide-spread and understandable fear of how drastic an embouchure change can be. It is usually quite difficult psychologically to have one’s playing ability reduced, even for short periods, while one undergoes an embouchure change. Those who ascribe to this philosophy are there for willing to accept, for example, the rolled-in lower lip or an asymmetric mouthpiece placement if the technique functions well, seemingly without major problems.
To conclude our post with more on the topic of upper lip placement, Wekre offers a bullet list of questions to complete the sentence “To what degree should the upper lip” and concludes that
a kind of combination of the natural and the slightly rolled in feel is what I recommend.
College is starting up again and for students new to college, this means a brand new environment and an unfamiliar horn teacher.
In the sidebar of the article Karen McCally outlines the original study that inspired their research which involved the use of a 
To briefly expand on the thoughts expressed in the post linked above the idea is, especially if you aspire to be a pro, to find a horn you think you can win a job on; a horn that produces a professional sound.