so far you are dead on. The amount of movement can be adjusted at the transmitter if you have a computer radio or it can be adjusted by linkage if you have a more conventional transmitter. The secret to mechanical adjustment is that you don't want the arm to start perpendicular to the control surface. the further along in the arc the servo horn is the less movement it will have available. in the arc of movement the maximum verticle displacement is when the servo arm is perpendicular to the control surface. As you complete more of the circle arc you get a greater percentage of lateral translation instead of the verticle translation, so less control throw hence forth the diferential.