A half square triangle square is cut 7/8" larger than the finished square. For example, your half square triangles finish at 8" so your starting square would be 8 7/8". Draw a line down the center on the wrong side, sew 1/4" on each side of the line, cut on the line, press toward the dark. The square SHOULD now measure 8 1/2" because of seam allowances.
BUT, I always cut mine to 9" and square them up. By this I mean, I would sew my pairs of triangles together and then trim them to a perfect 8 1/2" to finish at 8" after seam allowances. This is a lot more accurate and in the end you will be a lot happier with your quilt.