The objective for rigging the rudder with the correct amount of right rudder trim is to have no rudder pressure required at cruise air speed, either right or left.
At this setting, the rudder position just balances out the helical prop wash pressure on the left side of the rudder causing the plane to want to turn left.
However, at this configuration, some right rudder will still be required during climb and lower airspeeds, and left rudder will be required during higher speed descents.
This rudder bias for right turning pressure is typically accomplished with a trim tab on the rudder.
The trim tab on the trailing edge of the rudder is bent to the left causing the rudder to move a bit to the right. Then the tail moves to the left, giving right turning pressure to the plane.
Getting the right bend in the trim tab isn’t easy. Trying to do it on the plane is troublesome, and taking it off to guess at a new angle takes time and probably messes up the finish on the tab.
An easier and more precise way is to adjust the tensions in the top front fin tail wires. This will move the leading edge of the fin very slightly to the left, and accomplish the same thing as the trim tab.
First, fly the plane at cruise and see how far to the right the ball moves with no rudder pressure. At a half ball, try two turns, and a quarter ball or less, try one turn, as follows:
Leave the trim tab on the rudder.
Loosen the jam nuts on both top forward tail wires.
Using proper wire wrenches, with wooden centers, loosen the right hand tail wire two half turns. (one full turn). Try two turns for a half ball out.
Make sure that the threads on the wire still extend beyond the measuring hole in the clevis.
Tighten the left hand wire the same number of turns and lock all four jam nuts. This is a good time to check for the proper tensions in the tail wires.
Now fly the plane again at cruise and see where you are. It is better to sneak up on the correct setting.
If by chance you have to hold left rudder at cruise, then there is already too much correction in the rudder tab and/or the fin wires, and you will have to go the other way.