So if the longarmer wants the back 5" longer than the top, it should be +2.5" on each end of the backing? The way I'm doing it, the backing is pieced from straps and extra blocks, in vertical rolls, so the extra width is there, doesn't matter if it's a little more to the left or right. But I'd like to come close to centering it top to bottom. If I were going to quilt it myself, that'd be no problem, I'd figure it out, lol. But I'm sending it out. I'll ask the quilt artist, but I'd like to have a clue first.