This little drill works for me:
1) Decide the size of the quilt & figure the backing. EX: a crib size (45x60) would require about 3 yards for backing & binding.
2) Gather the fabrics for the top and buy enough of each to total the yardage of the backing fabric. EX: for 6 different fabrics in the top, I'd buy about a half yard of each---a total of 3 yards.
Of course, I make mental adjustments according to the pattern, but the total yardage in the top should be AT LEAST as much fabric as the backing. As others have said, buying a quarter yard extra here and there is how scrap stashes are created!