A worked answer
A twin top at 70 by 90 inches needs overhang on the back. Four inches each side is the default here — change it in the card if your longarmer asks for more.
With 4 inches of overhang the back is 78" × 98". On 42 inch fabric that is 5 1/2 yards. On 108 inch wide backing it is 2 3/4 yards.
The calculator tries both ways of laying panels and keeps the cheaper cut. Seam 2 widths across the length of the quilt. See also how much fabric for a twin quilt for binding and batting in the same size, and the general backing calculator for any inches.
A 70 by 90 inch top with 4 inches of overhang needs a back about 78" × 98". On 42 inch fabric that is 2 panels (5 1/2 yards). Wide 108 inch backing is usually 2 3/4 yards.
Wide backing removes seams when one panel covers the quilt. Compare the two yardages on this page before you buy.