The servo arm should of course be at 90° when centered so that the motion is symmetric, Ackerman serves a different purpose here. The rudder horns are intentionally not rotating around the exact center of the rudder hinges (instead it is further aft), so that the line that isn't pulling will give a slack. This way, it can be setup to have zero slack around the center position (where it is important) and a little slack when it doesn't matter, i.e. at large deflections.