This is my go to method for flying geese. You need no special rulers. If they have a size that I cannot cut on my ruler, I go up a bit on the squares. like 3 3/8 to 3 1/2, so just 1/8 diff. Making sure that the other square is appropriately sized up) When you are finished, you will have a bit bigger goose you can trim to the size you need for your pattern. Hope this helps!
http://www.patchpieces.com/files/flyinggeese.pdf
ps if you dont know what size you need? try making ONE and measure that. Pain I know..I have been there, but in the long run you will be happier.