Quiltler's full or double quilt is 85 by 90 inches. With 4 inches of overhang the back is 93 by 98. Ninety-three inches is more than two 42 inch widths (84) and less than three (126), so three panels is the 42 inch answer. The calculator tries both seam directions and keeps the cheaper cut. On 108 inch wide backing the 93 inch side fits in one panel, which is why wide goods earn their price on a full bed even more than on a twin.
Plan the top in the full quilt designer. Twin backing is smaller; queen backing is how much backing for a queen quilt. Binding for this size is how much binding for a full quilt. Diagonal seams rarely beat three straight panels at this width — check the diagonal backing calculator if you are curious.