What your seeing is correct.. The only way to make it not do that is to have the same distance between the rods on the servo as well as the tiller arm of the motor. Also the pivot point of the tiller arm is best over the center of the motor mount pivot.

Having said that most of the time you end up with about a 3to1 rate. The resin for this is that servos typically are not tough or strong enough to have a 1to1 rate.

You will be OK with what JD has stated.

Grimracer
Aquacraft Boat Designer