is the tail holding? Hover? Crosswinds? Flip? Backwards flight?
That's the most important thing.
How high you can set the gain can depend on:
Ball position on the servo arm
Slop in the tail and linkages
resistance in the tail and linkages
resistance to the tennis raquet effect
I like to have the ball moved in far enough so that I get full slider travel w/ as close to 100% servo travel as possible. To far in and you lose speed. Too far out and you lose servo resolution and torque.
I run a chinese weight mod to reduce load under rpm due to the tennis raquet effect.
If you use gyro sense, then the gain% is a value need to calculate from your setting cause the value is not linear to a % scale. Gear mode is kinda the same thing but many people reference gear mode value as the gain. That's what Dale is mentioning. Finless Bob has a video mentioning this somewhere.