Don't forget the other area for adjustment is where the clamp block attaches to the steering tube on the rudder end of the cable. Turn your wheel lock-to-lock to find the center of travel, then rotate the shortest direction to get the shaft keyway where you need it and lock it down in that position. Then loosen the clamp block & allow the steering tube to move fore & aft as you manually center the rudder. That'll get you a straight wheel and straight rudder, but you may end up with too much differential travel in either direction depending on how far you have to rotate the hub to center the keyway.
If so, you can pull the cable out of the helm (I'm assuming rotary?) and note the orientation of the hub when the cable is free, rotate the hub the desired amount to correct, and reinsert the cable. Could be a trial & error process, but another method. Drape some rags in the area - the helm end is a greasy mess.