If you work through the geometry, you'll see that in order to exactly duplicate the movement at the control surface it will need a round control horn. Otherwise the take-up side of the wheel pulls more cable than the other side needs reeled out.

Another solution is to modify the servo wheel to have a 'D' shape.

Some argue that a slack line on the feed side is no big deal, but I disagree since occasionally aerobatic planes can back up (tail-slide, stall turn).