I believe it all depends on the thickness of the batting you are using. An 80/20 batting will not take up as much fabric as thicker or heavier battings. I wouldn't be surprised if a 2" coordinating border all the way around the backing before you quilt would take care of the possible overlap. Also, the heavier the quilting the more it will take up the fabric. Don't worry yourself too much about this; enjoy the challenge.
