I love what you have done!
I like the backing you have chosen and would buy at least 4 yards for that size. I cut mine into two pieces that are 2 yards by 44 inches.... then I sew them back together along the 72 inch side so it ends up 72 x 88....I often also add an extra strip....if you have any leftover rails from the front, that can add a bit of flair to the back too!