I don't see a problem with it. You shouldn't run your red line diagonally through the plate. The plate seems to be just the right thickness so the index hole is even with the top of the plate and you should read the angle off where the scale also intersects the top of the plate, which in this case would be 17 degrees, then add 2 or 3 for resting the thing on the side of the bevel instead of horizontal, so this looks to be about 18-20 degrees.