When I need to buy backing and can buy a fabric wide enough that I won't need to seam it, it's an easy formula. 108" is wide enough for your 90" side. Then take the other measurement (108") and divide by 36" (1 yard). The answer is 3. 3 yards will be enough for your backing but you need more because the fabric will shift and you don't want to be short at one side, or corner, etc. I'd get 3 1/2 yards.... that will give you 9" extra on each end.... for shifting, etc. And you'll have some scraps for your stash..... Yeah!!