Retired math teacher answer: taking your 48"x 60", then adding 3" on all sides (plus 1" for the 1/2" piecing seam he allows) gives Wq = 55", Lq = 66", and Wf = 43". Now using those values in Mr. Flynn's formula, the length of fabric (Lq) needed is 92" (~2.5 yds). The diagonal seam will, of course, have the danger of stretching due to the bias edges. I highly recommend a good starch and pressing before cutting the backing. I've done with and without - with is worth the time and effort, believe me. I used to use this exact problem in geometry classes for "when am I ever going to use this in real life, Mrs. S" questions?