It should be quite clear that once framed a human figure standing into the square area which itself outlines, whatever figure you want to cir­cum­scribe to it with the feet in an extended position can never be tan­gent to the base of the square, that the feet themselves overstep.
This Perrault had tacitly hinted it, although he did not resolve the conflict (page 38-39) , while Leonardo had solved  it by not extending the feet.
I won't be so implied. Therefore the bonding of circumference and squ­are to the same body, can only be as­sumed only in two ways:
  1. or as in the Y case of page 54; since with the legs abducted (X), the ex­pand­ed feet would enlarge the circle (by navel) more than the hands.
  2. or in a virtuous tangentiality, keeping the feet together and resting on the ground, that is, not expressly extended, even if the interpretative dif­fer­ence between expanded and relaxed could reduce the gap.
In fact, while the hands open to their fullest in normal functions, the feet do not completely stretch in the usual ambulatory exercise, but just for athletic/aesthetics; anyhow this does not detract from the validity of the mea­sure­ment criterion of Vi­tru­vi­us, already corroborated in itself.

Reducing the feet side of the ultimate circumference, up to the land line obviously in­vo­lves a last step: the pin­point­ing, or better the conceiving the function, of the actual center.
One can well make out that the navel was flaw­less for its immediate sym­bol­ism as the center of the body, to the architect Vitruvius who did not know the Divine Proportion, and in lack of any other evidence. From here to figuring a circle around it, was a short step, triggering the his­tori­cal process of designing that effective circumference which he him­self didn't aim to draw, perhaps without realizing to challenge posterity.
But as to the navel, of what has been the 'feed­ing center' within the womb, after the first breath all that remains is only a passive ref­er­ence, noth­ing more than an inert scar, which means noth­ing for the rest of life, least of all a memory ;).