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.