If you want to use all the same fabric on the backing, then I suppose you are stuck with buying yardage. I never know just how big my quilt is going to be so my backing is figured off the measured size + a healthy chunk for overage and stash.
I have seen some nice back art and am working at the courage to use up stash for backing. So far my back art has been limited to a center panel flanked by coordinating fabric on the side.