The back is done with a 20" inch square of the "interest" fabric.
I had to fudge like there was no tomorrow because I forgot about the seam allowance.
I had to "make it work" because I had no fall-back fabric for the back of the cushion, but that meant stitching in part of the pattern for the patchwork squares on the front. Or should I say , shapes on the front, once I'd done my shaving they weren't squares anymore.