Add overage to all four sides

An editable 4-inch-per-side default adds 8 inches to both quilt dimensions. A 60 × 72 inch top therefore starts with a 68 × 80 inch backing rectangle.

Confirm the real requirement

Four inches is a conservative longarm-oriented convenience, not a universal rule. Measure the completed top and follow your quilter's frame, clamp, and loading requirements.

Then plan the panels

Usable width, join seam loss, print direction, and seam orientation determine purchase length. The quilt backing calculator compares valid panel directions; next check the batting size.