Also, if you ever are short of backing fabric, here's a way to diagonally piece it. For a quilt that really needed almost 3.5 yards, the calculator showed that I could get away with under 3. Not sure of the exact numbers, but it's worth looking at.
This site includes an on-line calculator and instructions that I was able to follow:
http://www.multi-patch.com/html/diag...calculator.php
Link to the originator's instructions and formula:
http://www.flynnquilt.com/workshop/FreeLessons/