Isn't it c a very crucial point? Vitruvius says yes! once drawn the square, based onto the man's height, the radius ch defines, or match­es the point k where the cir­cum­fer­ence – with the radius from the navel to feets om – must cross the square, and that's all is needed to supply several balances.

A further step rotates 90° the golden rectangle mchl and scale it up to resize the cm side (golden) to the full square side (cm+Φ2=cm red).
So the new y (red) vertical, is cutting the horizontal Φ point of cm.