Unless I am making a crib size quilt I measure the length of the quilt + 10" and double it. I cut that in half - seam it and then add 10" to the width and cut off the excess on both sides of the width...I often use that left over fabric to bind it. There is a handy booklet I have called: The quilter's Pocket Reference by Peggy Scholley. It has lots good information about how to figure quilting sizes.