Not sure if this will help, but if you lightly grip a large diameter paper straw (say 5mm- 7mm) between the teeth in the centre of your embouchure, do the ahh ooh, then release the breath through the straw – close the eyes, and that should give you a good idea of where the jaw needs to be while playing. I’ve also seen this done with a pencil, but I’m betting Greg doesn’t want you blowing against a sealed embouchure as that could promote tightness elsewhere.