I always cut my setting triangles bigger because sometime it is good to have a little breathing room between the quilt block and the border. Your idea won't work well because the edge of the quilt will be all bias. It will work for the corner triangles You need a much bigger square that you can cut diagonally into quarters.
Here is a popular website that might help you:
http://quiltville.com/onpointmath.shtml
I bought a ruler years ago and it was one of the best investments I made.