Backing too wide, not long enough, a Solution

I've been quilting for umpteen years and this is the first time this "trick" was useful.

My finished wall hanging is on the right, backing is on the left.  You can see that the backing is too wide, but not long enough.  I am aware I could take the extra side fabric and sew it along the bottom, but I wanted to try this trick.  The ruler is just sitting there - I'm trying to visualize the solution, but this first photo is NOT correct.  John Flynn  http://www.flynnquilt.com, has a solution but I'm ashamed to admit when I see his formulas, my eyes glaze over.  DH tried to "help" me, but I'm still not sure if he is pulling my leg, trying to buffalo me, or perhaps he really knows the solution.

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!)