When a TQ is laid flat, how wide should the bottom edge be so that when cinched the enclosed footbox is a circle around 15".

Is it as simple as (15 x 2) plus a little extra = 34" ???

This picture may explain what I am asking.

TQ.jpg

On a related note is 15" wide enuf for a size 11 shoe?
(I know its subjective, but I welcome any opinions)