I don't think I have heard anyone mention yet the fact that the mechanical advantage increases with the 1/cos() of the angle through which the servo arm turns. At the farthest most travel limits of the servo arm, the pushrod is moving less and less actual distance per degree of servo arm rotation. The region that is most linear is at 90 degrees to the pushrod. Once the servo arm has rotated 60 degrees, the mechanical advantage has increased from 1.00 to 2.00. When it has rotated 70 degrees, the mecahnical advantage goes up to 2.92. At 80 degrees, it is 5.75!