I draw a rectangle the size of the quilt top and write the measurements on the end and sides. It is best to have a seam going horizontally across the back of the quilt. If your quilt is 59 x 72 and the fabric is 44 inches wide,you would need two pieces 2 yards long, sewn together would be 83, the seam taken off. Subtract the 59 from the 83, which leaves 24 inches, cut off 12 from each side so the seam is in the middle of your quilt. For this you would need 4 yards of 44 inch fabric.
Years ago I was told by my mother you need 5 yards of 36 inch fabric to do a double sized quilt so if you see a good buy on fabric you would use someday for a backing, get 5 yards or a bit more if it is the end of the bolt. Today you won't find 36 inch fabric, you might find 54 or 60 inch wide. And you have extra for your stash.
I like to use the 1/4 inch graph paper to decide fabric needs, each square to represent a foot or a yard, depending on the project and needs. 11 squares would equal the 44 inch fabric we buy today, this is a good way to estimate your needs.