I think artists who have been animators and illustrators make great teachers because they have spent so much time simplifying and finding the essence of things. I’ve been watching Glenn Villpu and Steve Huston, and experimenting with the rule of thirds they teach on head proportions (first introduced to me by Mike Nolan).
This is usually stated two ways — either that the head is 3-1/3 units high (Loomis), with the bottom of the chin, base of nose, and brow line at unit marks and the forehead to top of head 1-1/3 units; or that the head is 3 units high, with even markers at the chin, base of nose, brows and crown of head. I have a big database of head photos, and looked at a lot of them with this in mind. I found that these two systems do cover the vast majority of heads, with somewhat more in the 3-1/3 category, so it makes sense to me to have both systems in mind. Hair often obscures the top proportion, of course.