pasithea

Waist measurement is circumference (c) Add 2 inches for seam allowances.

(c + 2) / (2 * pi) = r

Radius (r) is the amount you open your compass to draw an arc on the edge of the plastic for the skirt.

How long do you want your skirt (b) ?

How much of an angle do you want on your skirt? (A)

This is a right triangle, so we can subtract A from 90 and get B. (The angle at the bottom edge of the skirt) We also need to add the radius from above back in so: (b / sin (B) * sin (A)) + r = a ALSO (and more importantly) (b / sin(B) * sin (90)) + r = c

2a will be the overall width of your skirt at the bottom edge. Also, when determining your edging material, C = ( 2 * pi * (a -2)) will be the length of the material you need for edging (plus overlap if needed but I factored it out for the design we used)

c is the amount you need to open your compass to draw the lower arc of the skirt (from the same starting point as the waist measurement)

Structurally, 5 crimp-style snaps on either side seemed more than sufficient to hold the skirt in place. Put a metal post between the tines and push down to get them through the plastic. Otherwise, assemble normally.

Of course, this is the ultra-geeky approach. The waist measurement is the one that's really important. You can pretty-much eyeball the rest unless you enjoy being pendantic and anal or you just want to make the most optimal use of your materials.