I have on two occasions had to add about a 2" strip to the backing to have something to clamp to on the sides. If I do this for a customer (and yes, one was my own and I didn't get enough backing!) I do charge for the hassle of doing it--that also reminds them to give you at least 3-4" extra on all sides to attach to when long arming. I'd add that I always "float" the top and the batting (meaning I don't attach either to the roller, instead stitching it directly to the back at the top and then basting down the sides as I go)