Basically, that looks like a square in a square.
http://www.squareinasquare.com/
There is a video at that site that shows how to use the ruler. Perhaps if you watch the video of that technique you will get an idea of how to sew a triangle onto the side of the square and then trim it to fit.
http://www.quilterscache.com/S/SquareinSquareBlock.html
This lady has a technique that will help you immensely, I am thinking.
Hope these help. At least with the quilterscache.com ... you will be able to use what you have without an extra expenditure for another ruler.