Uh, there seems to be a bug in their software. I put in a 72 x 80 quilt and this is what I got:
Backing Width: 81.00
Backing Length: 88.00
Fabric Length Inches: -3,520.00
Fabric Length Yards: -97.78
I'm not a math whiz, but I'm pretty sure that is not the right answer! :-)