measure 3 places - at both ends and across the middle. Average the three measurements and use that size to cut your border. Pin at the quarters and ease in if you need to. If it appears to be too much to ease in, you have a different problem, perhaps your cutting or seam width. Maybe you can go back and stitch again at some of the side blocks. You should only have to go in a couple threads on a few and you can cut out quite a bit of extra. WITHIN REASON. If it is way off, you need to investigate further and find out where the problem is. But, don't be too hard on yourself, even the most experienced of quilters gets one that just won't fit!