Try this method. She tells you how to get the size you want with no wasting fabric. I will keep looking the original way ..putting a square on two corners, sewing the diagonal and trimming to see if I can find one that shows how to calculate size too...
http://www.patchpieces.com/files/flyinggeese.pdf
also, if you read what she wrote at the top of that tutorial, she says:'just remember that the width of the finished unit is twice the height'