Marsha McCloskey has a very accurate way to make half-square triangles. She calls it Bias-Strip Piecing.
"Each HST is made of two triangles but is actually cut as a square. In this method, large beginning squares of fabric are cut into bias strips, which are then sewn together. Squares are then each cut with the seam line centered corner to corner. Because seams are intially sewn on the bias edges, the cut squares have the straight grain on the outer edges."
A detailed description of her method is described in detail with pictures and cutting charts in many of her feathered star books. Perhaps you could find one in your library.