I have pieced the back of a quilt also. Use the width of your backing fabric right down the middle of your quilt vertically. Use leftover fabric from the front to sew large blocks together to make up the difference on either side of the strip down the middle. Let's say your quilt is 60" wide and your center piece has 42" of usable fabric (cut those selvedges off). You need at least 9" on either side to make up the difference. Sew blocks together to make up the 9" and sew one on each side of the center. My first thought was that I was being stingy not buying two lengths of fabric for the back, but after it was all done, it turned out great.