Count the length after the joins
A pile of binding strips isn't the same length as the binding once they're sewn together. Every join uses some fabric, so multiplying strip count by usable width can overstate what you'll have. The binding calculator subtracts joining losses before deciding whether the assembled strip is long enough.
Choose the join you plan to sew
For diagonal joins, QuiltMargins allows one strip width per join. For straight joins, it allows two seam allowances. These are the amounts used in this calculator's plan. If your joining method needs a different amount of trimming, check a sewn sample and make sure your planned binding still has enough length.
Eight strips have seven joins
With eight 40-inch strips, you start with 320 inches of fabric length. Seven diagonal joins in 2½-inch strips use 17½ inches, leaving 302½ inches of binding. A 60 × 80-inch quilt needs 280 inches around the edge, or 292 inches with a 12-inch finishing tail. The eight-strip plan covers that total.
Keep the finishing tail separate
The tail is extra length for joining the ends when you finish attaching the binding. It isn't another allowance at every strip join. Set it once in the tail field, then review the joined yield and target length together before cutting.