I could not make perfect half triangles until I purchased this ruler. Elenor Burns uses this ruler in her "Underground Railroad" quilt book. I bought the ruler at my LQS, however here is a link.
here is the link to it http://www.quiltknit.com/RU295-TRIAN...AY_p_2519.html
slashed line on sewed line according to size of block
finished perfect size square