The only ruler I use for 1/2 square triangles is the Wonder Ruler. They are so easy, and they come out perfect every time. I used to stay away from anything that had triangles before I got this ruler. Here is the link with a video on how to use it.
http://www.connectingthreads.com/Too...r__D81315.HTML
Click the tab that says bonus media, and they have a video you can watch.