Whenever we come up short it is a "design opportunity"! Do you have any extra blocks from the front of the quilt? Often quilters will incorporate the "left overs" into the backing. Also, piece a coordinating fabric right down the middle of the backing, everyone will talk about how clever you are and nobody will know you goofed on the backing...afterall, it was a design opportunity. Some of my mistakes and led to some of my best designs! That is what makes is so much fun, to me, anyway.