I would add the outside circumference (the length around the outside of the quilt) and cut that many inches from the width of fabric, adding 2.5 inches for each strip and one more 2.5 inch for the join. Yes, 2.5 is all you need for the join; you cut off the overlap at 2.5 inches. So the circumference is 322 inches. The usable fabric is 40 inches wide. 322 divided by 40 is 8.05 strips. So you add 8 times 2.5 plus one more 2.5 for all the joining seams and you get 22.5. Nine strips will be plenty. Nine times 2.5 is 22.5 inches. Add a little for squaring up the fabric and 24 inches should do it.
If you make your binding another width, substitute that width in the above formula.