You can't use it that way. It is limited (although apparently doesn't error out) to backing 150% of the fabric width. So you could only do 60" backing with your fabric width. You can however put in 80" fabric width (meaning, you seam it first). Then, the calculator gives 2.17 yards for this particular quilt - which means 2.17 yards 80" wide, or 4.34 yards of 40" fabric.
Quilt adict - your 7 yards seems too long, so maybe you made the same mistake? 7 yards would make a backing 80x126 with regular seaming. And note that you don't have to lay out 7 yards of fabric to cut diagonally. Cut in half, seam, then lay out 3.5 yards (by 80") to cut diagonally.