The MH-9 series (including AH-9 and M-900) all have incorrect RPM when the engine is running at normal conditions.
When the helicopter is on the ground with collective full down, N1 is at 84%, N2 is at 91%; there's no needle for rotor RPM on the in-game panel but if it's coupled with N2 then it should be at 91% as well.
But realistically speaking, both N1 and N2 should be aiming at 100%, the same goes for rotor RPM.
in-game N2 and rotor RPM:
The problem with the current ARMA model is that when you're making aggressive manoeuvres, the N2 will easily fall below the green zone and "RPM light" will go red.