I usually lay my finished quilt top on top of my backing fabric to measure the length. I put a pin in where my quilt top ends and bring it to my cutting table. I re- fold the backing fabric and I measure 6 inches past my pin and cut across the fabric. If I need it wider than 1 length of the fabric, I use the already cut piece of backing to cut more backing. The extra 6 inches gives me approx. 3 extra inches fudge room on both ends of the top in case it stretches or the quilting takes up more fabric. When my quilt is quilted, I square up my edges and the extra fudge fabric goes in my scrap bin.