Hi, I need help with my quilt back. I have a finished top that measures 58.5 X 70.5 and I am trying to piece the back with two fabrics pieced into 3 strips. When I purchased the fabric I told the lady what I was planning on doing and that I had a finished top measureing 60.25 X 72. She sold me 3 yards, two yards of color A and 1 yard of color B. She said I would have to cut color B and piece that into one long strip, which I did. She said I would have enough of color A in both length and width. What am I doing wrong because it seems that I won't have enough. Thank you.