44 X 36 = 1584 square inches
68 X 88 = 5984 sq inches
5984 \ 1584 = 3.78 yards for the top flat.
6 yards should be plenty even with opppps. As long as your pieces are not really small where you would need to add a percentage for seam allowances. The only other thing you need to determine is what percentage of each fabric you want the top to be.