Milan Yancich in A Practical Guide to French Horn Playing offers a practical approach to finding a good hand position.
There are many hand positions that result in a range of sound from a closed or covered one to an open sound….
There is no one correct position for the hand or the horn. A great deal depends on the sound one wants to produce. To aid the student in finding a good hand position, I instruct him first to hold his hand flat, fingers together, and slightly cupped, as if scooping water out of a brook to drink. I then tell him to insert the hand into the far side of the bell, far enough in so that if he were supporting the bell while standing, it would rest on the thumb and forefinger. He should then play a note with the hand wide open, and as he sustains the sound he should slowly close the hand until the tone becomes muffled. At that point he should open the hand slightly by moving the hand out a bit. That will be his approximate hand position.
Most great horn players feel quite strongly about hand position in the bell. One of my colleagues told me that if his students were errant in keeping the hand firmly in the bell he had them place adhesive tape inside the bell to make sure that the hand remained quietly in the bell in the same position. My cure … is to stand while playing. Since the hand must then support the weight of the bell, it cannot easily be moved.
Yancich does not suggest adjusting the bell from note to note as it “can lead to an uneven scale.”
A critical factor: the stopped horn/open horn relationship
William C. Robinson suggests in An Illustrated Advanced Method for French Horn Playing that to find the best right hand position one first find a good stopped position for the right hand in the bell.
When a good stopped position is attained, the heel of the hand should be swung open, with the seal maintained near the ends of the fingers. This should result in a good right position; the hand can be opened or closed slightly, according to individual tonal preference.
Personally, this is the angle I most often take with adult students to find a good hand position; you want to ideally be able to go quickly and easily from open to stopped and back to open by just closing and opening the door of the hand in the manner Robinson describes. Of course this won’t work with every student, especially if they have small hands and a large bell, but it is an ideal if it can be achieved.