/ic/ - Artwork/Critique

Please critique on how accurate the perspective for this head is. Does anyone know by how much the top half of the front plane shortens as a person tilts there head back at various angles, or if the person tilts their head downwards, by how much does the bottom part of the front plane shorten(30^, 45^. 60 etc.)? As of now, I know that the ear stays in the same spot (halfway point of the head), so I draw lines from the top of the ears and estimate the point of where the eye line is and compare that to the angle of the top of the head.