I might be going into too much detail, but I think the line that joins the "S" and "G" in the slope locator is 30 degrees downward, because if you pay attention to the yellow "down" arrow, there's a pattern. Every two pixels the arrow moves to the right, the next time the arrow moves, it goes down one pixel. This is actually the ratio of the hypotenuse to the vertical leg of this invisible triangle. Take the sine inverse of the ratio and you get 30 degrees.