Couple of questions. Is 13.5" the finished block size? If so, they are about 19" across when set on point.
My next question: is the white area part of the block, or is that wide sashing with cornerstones. If it's sashing, how wide are you going to make yours? This is going to affect the overall size and until you decide on this, you can't determine how many blocks you will need.
Finally, here is the chart for the setting triangles:
http://quiltville.com/onpointmath.shtml