Formulas sometimes can be confusing. As far as I know there’re for example for the proportions of a realistic head basically two methods when drawing:
1) Draw an egg shape and divide into two halves horizontally and vertically:
The eyes are on or close to the horizontal line: Some authors place the eyeballs a little bit higher or a little bit lower than this line, others place them exactly on the line.
2) Draw a circle for the cranial mass of the skull and divide it as the egg shape above. The eyes are on the horizontal line. But now you have to extend this circle downwards to get an egg shape. Therefore the eyes are placed higher than with method 1.
I think, that these formulas can give a starting point as a beginner but you have to leave them behind in order to grow as an artist. Your also have to be aware of the discrepancies of the different methods and even within these methods depending on which teacher you’re following. Following formulas too much can also take away the fun which I think should be always there in order to keep you motivated.
