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.
The Loomis idea that the eye bisects the head only works well on the 3-1/3 types. On the equal thirds, it’s better to follow that the center of the eye generally falls 1/3 below the brow. Another common marker given is that the bottom of the bottom lip is halfway between the two lower marks (Villpu and Huston). This is generally true, and it’s pretty easy to spot variance. The placement of ears is all over the map.
Some people (see far right figure) don’t work in either of the systems; among the outliers, the most common variant is to have the middle section be the smallest. (That guy is really unusual with the short bottom section.) Within the two main systems, the most common sub-variation is for there to be a slight increase in the distances from bottom to top, that is chin to base of nose is slightly smaller than nose to brow.
A very quick check with a piece of paper with three even marks tells you what kind of head you’re working with; just tilt it at an angle until the marks line up with features. By the way, “brow” is the hardest to identify; I find that the top of the eyebrow is most useful, at least in profile. Loomis’s drawing shows that marker kind of middle of brow, but for me it’s working out a bit higher.