Basically, where should my TX flat throttle curve be at to adequately maintain my desired headspeed?

For our governor it does not matter - what really matter is if your motor has enough physical room to hold the head speed.
In governor settings you can set Max. Head Speed which determine what head speed will be at 100% TC.

If you know that your helicopter can handle for example 3000 RPM maximally, you can set this value as Max. Head Speed.
85% TC should work well.

Example: If you will set 75% TC, you will get 3000 * 0.75 = 2250 RPM. There is a lot of room for good holding.

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Spirit System developer