Phil is correct on this one. The gain is used for holding power. Typically you want as much mechanical gain as possible so that you don't wear your rudder servo out as fast when using gobs of electronic gain. Electronic gain makes the servo hunt for center alot harder and this send ALOT of corrective signals to the rudder servo. In this scenario you will reduce the life of your rudder servo.
So, you can improve mechanical gain in two ways:
1) Altering the distances on the rudder servo arm ball link location and/or the rudder bellcrank ball location. On the Century Helis they only give you one hole on the rudder bellcrank so that leaves you with the rudder servo. Further out on the arm increases the gain and vice versa for lowering the gain.
2) Tail blade size/airfoil. A larger tail blade will also increase the effectiveness of the tail and make corrective inputs less criticle. But, there is a point where you can over do it. Some tail rotor hubs are not capable of being stressed by larger heavier grips than what the manufacturer designed to for. My findings are that the Century tail rotor hub on the SE and Raven series is tuff enough for anything you throw at it.
The way I set my gyro is like this:
1) Do the initial setup with 90 degrees at the pushrod/servo arm with the tail rotor pitch slider centered in its travel.
2) Hover the heli and trim the model in NON heading hold using electronic trim(Century tail works best this way. I ususally trim them out mechanically on my other helis). Then match your trim in all your flight modes if you are doing IDLE UP flying. Activate heading hold to make sure you have no drifting in all flight modes. If you do, then your electronic trim is not matched in that flight mode to the one you trimmed the heli in. On the 401 if the light blinks then it ain't right!
3) Now turn the gain up in a hover till it wags. Then back off 5%.
4) Fly the heli in FF work the tail back & forth to see if it wags or oscillates. Turn the gain down till you can fly FF and not wag. Backwards flight is great way too.
5) Now, in a hover work the tail back and forth and see it it stops on a dime. If it doesn't use the "Delay" pot on the 401 gyro sensor till it does.
This will get you set pretty close. Do some flips/tumbles and see if the tail or nose point elsewhere as you go around. When I started flips and tumbles using a 401 and heading hold activated I would flip over to inverted and the nose would be pointing at my right shoulder. It took some tail rotor blade selection & gyro setting playing but I got the tail on my Falcon SE V2 holding pretty good. It's not perfect but pretty good. The torque tube will help alot.
Now, if you have a Raven 50 and are doing tumbles or flips and the tail is walking with the gain turned up then you might need better tail blades(doubtful because the fiber filled Century ones are awesome) or a larger tail rotor disk.