I have done John Flynn's method, successfully. I had a piece of fabric that was a bit too long and a bit too narrow, so I cut it diagonally from top left corner to bottom right, slid the 2 cut pieces in opposite directions until I had the width I needed and still had enough length. I marked the 2 pieces, sewed them back together, starting and ending at the marks, cut off the end triangles, i.e., squared up and, voila, I had the rectangle the size I needed. This was for a queen sized quilt.