Hi Paul
There is no set ” rule” for this just trial and error and it will come to you, a good start is see where the reed starts to separate from the mouthpiece at the tip (opening) and put your lips in line with that? you will soon grasp how much to put in your mouth by the sound you are happy with.
Paul, at the beginning it’s anywhere from one half inch to an inch. if you’re closer to the tip than that it will contrict the reed too much. if you get wa beyond that you’ll feel that you have very little or no control so you will determine the exact location yourself just by trying it.
as we progress the trick is to get a little more of the mp in the mouth because doing so will give a bigger sound…but this also makes it harder to play so don’t go too far until your embouchure is well developed.
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