I have cut the backing on the diagonal ala John Flynn when it was a bit too short. Here is a link for his site:
http://flynnquilt.com/media/uploads/...onalpb1103.pdf and here is a link that simplifies the explanation.
With the length as the issue, I swapped the way I worked with it around. I used it the way he wrote it when it was too narrow. (I played with a piece of paper first to work out the technique.