I always make my half square triangles by making the two squares of fabric that I'm using 1" larger than my finished square will be and then trim it down. This method you draw a line from corner to corner and sew 1/4" on either side of the line and then cut on the line. When you open up these two pieces you have two half square triangle squares that need to be trimmed to the right size. Usually it's just shaving a bit off each edge. There are two ways I trim. One is to open the square up and press it. Then I use a square up ruler and put the 45 degree line on the seam line and the measurement line of how big the finished square will be near the edge and trim accordingly. Quilt in a Day also has a ruler to trim half square triangles. With this ruler you don't press your square open first but rather put the ruler marking along the seam line and trim the two raw edges. With this ruler you are only making two cuts instead of 4 with a square up ruler and it is easier to understand the markings. It is a square ruler with many different size triangle markings. A good investment.