You do not have to change Rudder Common Gain, it is only multiplicator factor of the Gyro Gain that is configured in your transmitter.
For bigger helicopter you can leave it around 1.4x. But Gyro Gain must be always configured properly.
I recommend to put ball linkage on the servo arm closer to the center.
Can you share your settings please? You can attach here your 4DS file.
Spirit pro tail issue
Spirit System developer