here is a handy link for use mathmatically challenged mamas. TI seems like someone posted it on Amity a while back, Catabella maybe?

[url=http://www.csgnetwork.com/circlecalc.html0Circ el calculator)

Plug your waist measurement (or hip or what ever) into the circumfrence box and it will tell you the radius you need to draw your circle. Just add what you what length you want to the radious to draw the hem. easy peasy.