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:
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.
- 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.
- 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.
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 ;).