You first need to figure out how long your border will be. Then, divide to get an odd number of triangles (because you would want the ends to be the same colour. Let's say that your edge is 39 inches--that would give you 13 of one colour and 11 of the other. The edge of the triangle that touches the top would finish at 3 inches. You could cut squares at 3 7/8 inches and cut diagonally, but I think it would be better to cut a square at 7 1/4 inches and cut diagonally both ways. That way you don't have any bias on the edges. I am pretty sure that Quilterscache has something like this. I'm also pretty sure that I have it in a book someplace.