I have a question regarding lower jaw position when forming an embouchure. I'm versed in the mantra that you should be able to fit a mouthpiece shank / pinky finger between the upper and lower teeth, but I'm curious how "rigid" this instruction is vs. whether it's more of a "ballpark" goal.

I find that with my jaw dropped that much, my initial articulation after bringing the horn up is inaccurate, "flubby", and just generally not what I want to hear. Once I'm "set" after that initial attack, it's fine - but I've been working for months to get set on that initial attack and I'm at the point where my tongue is almost quivering at the thought of starting the note.

But if I let the jaw close a little, my embouchure feels more comfortable and secure. I don't hear any difference in tone, nor do others who I've asked to listen. I find that my throat shape is far more of an influence on tone quality than how open my jaw is.

Thoughts / concerns?

Consider it a "ballpark". When I taught students with clenched jaws I had them put a pencil between the teeth. Or I provided a piece of ¼ inch (6 mm) polyurethane squeegee material to bite on (don't swallow!).

Lower register can require a more open jaw and higher register requires a more closed jaw. When do you have to open/close? That's a matter for practice; but the amount of movement should always be minimized.

If it sounds and feels good, don't force an arbitrary position.

Definitely "ballpark," if for no other reason than natural human variance in the size, shape, and angle of teeth and jaws.

A better "ballpark" is to start with front teeth touching and open just enough to get a good sound.

Memorize the feel of that position and use it for as much of your range as possible.