Hey there! Not sure if this helps, but when I have looked at character sheets, it seems that people use straight lines to make sure each facial landmark remains in the same position.
I think I've seen those too, never thought it was that important, gonna have to try that. In another topic, how do comics artists keep character consistency between panels since they can't use this?
I'm not quite certain, but if I had to guess, I would assume they measure the distance between the main facial landmarks for the character (e.g. nose, eyes, ears, mouth) and then make sure that it matches proportionally. For example, if a character's mouth is halfway between the nose and the chin, they make sure that it is halfway in any angle they draw. Just my guess though, so take it with a grain of salt.