I always make my squares for half square triangles 1" larger than the finished block and then trim. I have two rulers that I use depending on which one I find first. One is a Quilt in a Day ruler that is square but you trim your half square triangle with it before you open it up so there are fewer cuts. Lay the marking on the seam line and cut the two exposed edges to square the triangle. The other method is to press the half square triangle square open and then place a square ruler with the 45 degree angle line running along the seam line. Get it as near two edges as you can with fabric under the ruler and trim the excess. Turn the square around and do the same to the two remaining sides except this time put the markings of the ruler that you want your half square triangle to be along the edges you just cut, again with the 45 degree line on the ruler along the seam line and trim the excess off the two remaining edges. With either of these methods there isn't much overage to trim, usually just slivers but you end up with true half square triangles every time. My avatar is a quilt made almost entirely of half square triangles which I did this way.