I don't usually bother anymore with the math. I buy backing material 104" to 108" and for 60 x 70 I would buy two yards. I would have plenty for the quilt with some left over for appliques, or a smaller Drag Around Quilt. Backing is usually 8.50 to 12.00. ( unless you go bonkers on the high price backing ) I saw some the other day, on line, that was 125.00 per yard. Yikes ! not me !