OMG! Do you have the 20 1/2" square Omnigrip ruler? If so, look at the size of block you are cutting. Whether it be 12" or 20". There are numbers at the point in the ruler that tell you what size to cut. The larger number is the one you use for the side triangles because you cut it on the diagonal both ways. The smaller number is for the corners, as you only cut it once on the diagonal.
Does that make sense. I was so excited when I was told what those numbers meant. I never can remember what size to cut either. Right now it is only on the 20 1/2" ruler. They are making rulers now that will have it on the smaller ones.
If you tell me what size your block is I can tell you what size to cut. (As long as the center isn't larger than 20 1/2")