In his head constructions Dürer always held to the tripartite division of the head described by the architect, engineer, and military master builder Marcus Vitruvius Pollio of Rome (1st century B.C.).  It is part of a canon of human proportions from Vitruvius’s Ten Books on Architecture.

“For the human body is so designed by nature that the face, from the chin to the top of the forehead and the lowest roots of the hair, is a tenth part of the whole height; (…) the head from the chin to the crown is an eighth (…). If we take the height of the face itself the distance from the bottom of the chin to the under side of the nostrils is one third of it; the nose from the under side of the nostrils to a line between the eyebrows is the same; from there to the lowest roots of the hair is also a third, comprising the forehead.” 

VITRUVIUS, Zehn Bücher über Architektur (De Architectura libri decem), Book 3, chapter 1, transl. into German and with commentary by Curt Fensterbusch (Darmstadt, 1996), 91–2. [English: Vitruvius, The Ten Books on Architecture, trans. Morris Hickey Morgan (Cambridge, 1914), 3.1.2. 

In Vitruvius’s well-proportioned men (homo bene figuratus) the head correlated to an eighth of the body length and the face was a tenth of it. According to this calculation, the crown of the head is a fifth of the whole head. Dürer varied the relationship of face length and head size and thus the height of the crown of the head. In the Head of a Man as well other relationships can be discerned.