These are not half square triangles so that is why the 2 pieces are not the same size. It you go to Bonnie Hunter's Celtic Solstice quilt she talks about the flip and sew method,also some info on her new leader and ender project.
Another way to do this is with inklingo. Here's the link for that site:
http://inklingo.com/shop/lucy-boston...63467c9dae521b
Good luck, I hate that flip and sew method
cindy