RE: Correct Mechanical Advantage
Use the maximum throw on the servos. This is a transmitter setting, typicallly called travel adj (on JR). JR Radioa express this as a percentage. From the factory, the transmitter is set to 100%. Change this to 125 or 140%.
Use a short arm on the servo or use one of the holes near the axis.
Use a long arm on the movable surface.
set the length of the pushrod so that it is at 90 degrees to the arms on both ends.
If you have enough movement of the surface, you are done. If however, you don't have enough movement, you must compromise some mechanical advantage for more movement. Put a longer arm on the servo (or use a hole further from the axis)