I have not read all the posts. However,,,,,,John Flynn has a technique that he cuts the backing diagonally for the back. the instructions are on his web site. I will try and find it for you. The formula will tell you how much to add to the length. My guess is a piece about 95". You cut from corner to corner and then slide one side dowm until the piece is the width you need. Then sew the seam and you will be able to save fabric and (according to him) have a stronger seam for the back. Let me know if you need the site. I use it and it works. Sure saves fabric.