Many years ago, I attended a sculpture workshop with a French artist called Frederick Chepeaux, who was a most beautiful portrait sculptor. He used eye size as a measurement for all facial proportions.
I use the same method when life drawing. I don't rigidly adhere to the measurements but they are very useful tool for checking likeness.
Using a serious face full on as my example, the widest part of the face is the eye level. 5 eyes wide. The nose is 1 eye wide.
The space between the base of the nose and the upper lip is 1 eye height. The lips are 1 eye height. The bottom lip to the chin is 1 eye long.
The upper base (where it attaches) of the ear lines up with the eye line, the lower base of the ear lines with the nose. The corners of the mouth (in repose) are directly in line with the pupils.
He had a similar method for figure measurement where he used the head height and width to give the proportions of the whole body but I haven't remembered that so well.
He had reference books on his methods. Perhaps someone else has heard of them.
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