2/3 upper and 1/3 lower is where most teachers would start a pupil but there is much variation seen amongst very good players. The Art of French Horn Playing by Phillip Farkas covers embouchure fairly well.
My instinct would be to advise that you use a mouthpiece with a similar internal diameter to that which you used on trumpet. Assuming you are in the US, then you will probably have a narrow rim mouthpiece and may find a wider rim easier - see
for examples.
Some issues to watch:
1. Dont force your lips within the rim of the mouthpiece let the mouthpiece rest on your lips - most commonly with the inside of the rim just above the red of the upper lip. Don't worry if this means that the rim is set into the lower lip - although if you can't get enough of the lower lip in comfortably you will have to set the rim into both lips.
2. Avoid pressure - especially if you set the rim into the lips. With a thin rim mouthpiece you will have no endurance if you use pressure.
3. Avoid stretching the lips to achieve higher notes. Tense them as if you are whistling. Farkas uses the expression "puckered smile" to describe the compromise.
4. Once you can play any notes in the top octave focus on the shape of the lip opening. This is important for tone and needs to be oval.
5. Depending on the trumpet mouthpiece you used, you may find that horn needs more air flow especially in the top octave. A shallower mouthpiece and narrower bore may be desirable.
6. Initially you should give priority to developing tone, a revised sense of pitch etc up to written third space C (and down up to 2 octaves). You will need a double, compensating or Bb horn to play reliably in the top octave.
