I understand that starting with 50% rudder gain in the transmitter and 1.00x Rudder Common Gain multiplier on the software is a good point to start for a maiden flight without worrying that the helicopter will fall off the sky. However, when it comes to tuning, how do I determine whether to increase the Rudder Gain vs. the Rudder Common Gain multiplier?

Rudder Gain should be your furst choice since it gives you kmore "resolution": a change of 1 will be a change of 1.

Once (if) you get at 100 and you need more you will need to use the multiplier to be able to achieve higher-than-100 values. In that case, a 110 multiplier (or 1.1, however it is called) will make your values 1.1 times, so for example 70 will become 77 (so you can use 92 as 101) and so on.

As simple of that, don't overthink it.

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This!