I measure the center, then the edges. If there is a lot of difference I use an average. If the difference is slight I use the center measurement. As I get better the difference is less. I do short sides first, then long sides. Similar to pt quilts and patchsamkim. I like to cut my borders from 1 continuous piece but if that means buying a LOT of extra yardage I will piece them, unless I know I can use the extra fabric later.