This question pops up from time to time and it always baffles me as to what the problem is.
Is everyone trying to cut the borders to the exact length before attaching to top. Why? To save yardage?
Some suggest taking several measurements across the quilt and averaging this for length of border- then easing either the top or border fabric to fit. What? Is this an attempt at "squaring up?" Or some other reason? Way too many steps and measurements where things could go bad.
Besides being the desired width, the border fabric only has to be one thing- long enough to fit. I only measure the top along the edge the border will attach to, and cut my border piece a couple inches longer. After it's sewn on opposing sides, trim to fit to the raw edge. If you've sewn on the long sides first you can now measure across the ends and cut your border long, and trim to width. The border pieces were cut to width, but after that I never pull out a tape measure. I take the border piece lay it on the top to see if it's long enough. If so, attach it and trim to fit. If not, lengthen the border piece. Works every time.
Am I missing something by not following convention?