actually, i've heard that Leonardo had to do a lot of faking to make his diagrams the way he wanted them.  he was more interested in mathematical ideals than pure reality.  this shows in a lot of Renaissance work in general.  i had a course once on visual art and perception, and they showed how lots of things we think of as "correct" in terms of perspective are actually egregiously wrong, but a convention in visual art.  and that a lot of correct stuff will look wrong just because certain POVs are generally avoided in media.

if i had to count heads, i'd do it on a photo reference, not based on an art book or an artist's diagrams.  a first generation copy is always better than the third or fourth.  but then, i think it's much better to acheive realism through being specific and not guessing at "averages," because i think that we (as your average modern citizens) spend much more time looking at manipulated images (and postworked people)  than reality and don't have a strong innate notion of average.