My solution was to calculate square inches. The wall hanging was a bit over 504. The backing was 610 - waaay more than I needed. Here goes. I drew a line from corner to corner, slid one side down, the other side up, and sewed it together with a diagonal seam. Voila! It's tight, but it works. The points will be trimmed off.
I've heard that long-arm machine quilters LOVE this diagonal seam trick as it spreads out that "lump" on the backing as it is rolled.
If you are interested in the formula, it's on John Flynn's page, under Instructions. Perhaps you are better at algebra than I. (Should I admit that the ONLY time I was sent to the High School Principal's office was in my Algebra class? I still have nightmares!)