there is an online quilting store that sells a set of rulers that automatically figure the additional amount to cut on half square triangles and quarter square triangles. You just set your ruler up with the correct set of triangles you want to make and cut the triangles the exact measurement you want. The ruler adds the additional inches you need for the finished product. I've included the link. My squares come out perfect every time.
http://www.guidelines4quilting.com/