A retract servo is not proportional. It is designed to go from point "A" to Point "B" at a given command. In other words, your Tx does not tell it how far to move, it just gives a comand to "move" and the servo takes over and moves to it's opposite end at its own pace.

So and electronic gizmo wouldn't really work. Now maybe you could reduce the voltage and slow down the motor, but then you'd also be losing power, which is something you DON'T want in a retract servo.