Yes, that's the method and wesing's post is the one I had in mind. Thank you all so much. Oh, the math, but the second link in wesing's old post has a calculator for non math minded people like me. Unfortunately, this method won't work for my quilt backing. I've got an 84x84 and I prefer 4 extra inches on each side, although I could do 3, but my fabric is only 42 WOF.
Oh well, I thought I'd give this method a try, but I'm saving this information and will try it on another quilt. Thanks again!