If you consistently come up with 2 different sizes, you probably are positioning your ruler differently when you cut some of the blocks. I keep typing and deleting... This is hard to explain! When you place the triangle ruler on your tube, note what the measurement is on the bottom of the ruler along the edge of the fabric. You can put a piece of masking tape on the ruler to mark it. make your first 2 cuts to cut the first triangle. Flip your ruler 180 degrees and position it so that the top of the fabric is the same measurement as your first triangle cut. Unless you have issues with your seam allowance varying widely, you can use the edge of the fabric to position the ruler.