Instead of using rulers, consider purchasing a CD entitled: Triangulations 3.0. I just purchased it and you print out the size you want, sew on the lines, and cut apart. The CD was about $30 but well worth it. Your half-square triangles come out perfect.

My husband made me one. He took plexyglass and cut it 12" long and 1/2 inch wide. Then scored the middle and took a magic marker and blackened the score. Works great.