Lets say your quilt is 90"x108". Measure around which is 396" Add 20 inches for corners and overaps and joining That is then 416". Then measure the width of your fabric - say 42". Divide 42 into 416 and you get 9.9 strips or 10 strips. Then multiple the 10 by the width of your strips - lets say you will cut at 2.5".
2.5" x 10 = 25" of fabric. I would add an inch or two in order to straighten up the fabric.