The rumble strength on GameCube controllers isn't controllable - it is either on or off.
In case the Mayflash driver is modifying the rumble signal, Constant 100% should be the maximum strength. If that does not work, try changing the slider to either extremes.
In case the Mayflash driver is modifying the rumble signal, Constant 100% should be the maximum strength. If that does not work, try changing the slider to either extremes.