I have an angle gauge that I picked up from the hardware store.. it's about six inches square, with a round dial in the middle of it to indicate the angle of the surface that the gauge is sitting on. On a flat horizontal surface, it measures zero, and on a vertical surface, it's 90. I wish that it had a "datum" function that would let you zero it at any particular angle, then read increments from there.... but for control surface throws, it's easy to take whatever angle it may be sitting at as the baseline, then just make sure that both surfaces move the same distance from that point.