Accoring to the iphone Quilt Calc using 43" inch fabric with a 52" wide x 70" length it will take 4 5/8 yards this allows for a 6" overage. To piece the back, either piece in rows (width) or columns (length) at a standard block size then add fabric backing until the coreect size if obtained!
P.S. I absolutely love this apps - I work in a fabric store and it comes in so handy can even calculate multiple borders!