If you can't sort it by adjusting the linkage and levers, have a look at adjusting the endpoints/rates in the Tx settings.
Most computer transmitters have adjustments for servo travel. By default it is set to 100% either side of centre. On my Futaba, I can set the travel between 0% and 140%, independently for each direction. To get equal travel on the boat I'm currently building, I have set one direction to 90%, the other to 140%. I couldn't fix it using geometry because the tiller would hit the hull in one direction if I set the tiller at right angles to the linkage, and the servo mounting plate wouldn't allow me to move the servo any further inboard without a complete redesign.