was your circle skirt too big when you were finished? did you cheat? tell the truth. did you try to skip the math part and do this:
say you wanted the waist hole to be 32 inches. cutting the circle can be a real bitch. so you fold the fabric in half twice, and then you divide that measurement by 4. this gives you 8 inches. you measure a line that's 8 inches long, because lines are easier. but then you make a curve, because you really need a circle. if you look at the gap between your 8 inch line and the curved line, there are several inches of extra space in there. now multiply that space time 4, and that's where you got a skirt that's too big.
if you really want to skip most of the math, get yourself a bendable ruler. that way, you can make the ruler curved and use that instead of all those calculations.