So many times I have measured for backing fabric and been inches short, here is what I do Before I sew it to the quilt back, I plan borders on the back of the quilt, guess what they add interest and look god when you fold one edge back on the bed, in fact I do this a lot. I put the border on all 4 sides and it looks like it was planned. BUT I always figure out the yardage first. Think the suggestion for adding borders top and bottom would also work for your dilema.