I'm trying to make a bag with a circular bottom, and I think I may have bitten off more than I can chew. I Don't know how to measure to make the circle the right size and the piece I cut is too big. I'm scared to cut it again, because if it's too small, I don't think I'm going to have enough fabric to cut another circle. Getting more is out of the question, since I am using these awesome upholstery swatches I found a a thrift store. Has anyone ever made a round-bottom bag? How do I measure to get the circle right? I'm terrible at math, so please explain carefully! If I can get the circle the right size, I think I can manage from there. Please help!!!!