as a long arm quilter, i prefer a seam from side to side, and not up and down, and i like my backing seams deep, at least 1 inch, and press the seam open. when i get fabric for backing, i add up to 6 inches on both sides, you dont want to be short, so if your quilt is 70 wide 82 inches is what i would get, and you have to add that twice, and if your quilt is longer than 88, you have to add it 3x....for a total of 246 inches =alsew, 2 seams in the back, so a total of 7 yards would be enough.
good luck
katz