I guess that I didn't understand this , Why would I want a diagonal backing, When I figured it out for a 65x 85 quilt with 3" on each side it came up with 8.6 yard. Seems a lot to me.I can use 6 yards by just piecing two 2.5 yard lengths together vertically. I know that I am not great at math but I drew this out and I think that 6 yards should do it. Unless there is a reason to use a diagonal.