As for the legs,let me stress again that arms and legs rotation have four own centers, far from the navel; and the circumference theory is not fully sustainable, unless it could be proved to encompass the four modes of wider extension of the body limbs – or it wouldn't be what is described.
See also this illustration of the "Vitruvian Man" in the edition of ‘De Architectura by Vitruvius’ by Cesare Cesariano: even if it doesn't match properly our scope, it clearly expresses the need conforming to the major body boundaries.
Where the text states unequivocally “circumference would tangentially touch the fingers and toes”. the toes are to be considered the lower extremities of the whole body, instead of the sole of the foot. That is the reason for specifying 'supine', where the feet and toes are extensions of the body, not buffers or bumpers of its height; or why to put it on his back, face upward?.All the more, only the second description, about the figure of the square, reads “plane of the feet”, which is instead the solution practiced once for all by Leonardo Da Vinci. In the Leonardo's rendering – in spite of this unnatural posture – the feet are not extensive, but always kept flat on the floor. Nor will this be enough to reduce them within the circle.