The math isn't really that impossible. For example, if your border is a 3 part one.... maybe 1", 2", 3".... just add the amounts together and add twice that amount to the length of your border (once you have stitched the 3 parts together). Remember.... when putting the side borders on, it will be a border's width longer at the top and bottom. And, I always add about 4" more just to be on the safe side. Since I use my scraps for applique, I never have any waste.