How big is your quilt gonna be? If the longest side is less than 100" this would allow you to place the long side of the top on the backing fabric selvedge to selvedge and have at least 4-5" on each edge, so you would take the short side measurement, add 8 or 10" and buy that amount. If the longest side is longer than 100", you will need to lay the top length-wise, so take the longest measurement, add 8 or 10" and buy that amount. I would probably buy a quarter yard more to allow for straightening