If your quilt is 71 by 71 and you are buying wide back at 108...
Buy 80 inches , which is exactly 2 1/4 yards....which gives you the extra 4 inches top and bottom if you are longarming...
you will also have over 30 inches by 80 inches leftover for one of the sides, which will cover binding...
if if you are buying 44 inch...buy 2 lengths...joining the two with one seam...this will give you at least 80 inches of width...so extra for longarming...that works out to 4 1/2 yards of backing only for the backing...
then add about 1/2 for binding...and more specifically...
you need 300 inches of binding...roughly...so that works out to 8 strips..if it is 2.5 inches wide you are using...you need 20 inches...just over 1/2 yard...as 18 inches is a yard....maybe best to get a little more in case of booboos... Hope this helps...