I recently saw a great idea for cutting triangles without the need for a 'special' ruler.
Cut a template from thin card and stick it onto a ruler with one of the sides of the triangle at the edge of the ruler. This is the edge you will always use to make your cut. Ideally, use a ruler that's not too much bigger than the template so as to make it easier to handle. 6"sq or 12"x6" etc in this case perhaps.
Now use your template as you would normally to cut triangles from a strip of fabric, but because it is stuck to the ruler you have the edge to cut against. If you see what I mean. For each triangle you just turn the template so the base of the triangle is either at the top or bottom of the strip, turning the ruler over each time.
This is much harder to describe than do I'm afraid, but once you get the template stuck onto a ruler I'm sure you'll see how this works.
I'm not sure where I saw this method. If it was from anyone on here, I'm sorry for not acknowledging you.