Tom,
I knew you’d ask a hard question. My math isn’t much better that yours.
If I did it right you will get a 22-23 inch cage if you use 13 Squares.
13 x 6” = 78” circumference 78” / 3.14 = 24.82” diameter. Take off a few inches of the circumference to account for the end being cut and folded over and you’ll likely get a 22 to 23 inch cage.