Autorotations and confined areas in H369

We were attempting to see if we could change the autorotation characteristics of the H369, by varying the speed and RPM.
In the manual it says:
"to reduce the rate of descent or extend the gliding distance, operate at minimum rpm - maintain rotor between 420 and 523 by use of collective control."
To do this, and to keep the engine from coming back in (which would mean you return to powered flight) you must close the throttle to idle. Then you can play with collective pitch.

It also explains: - rotor rpm is 485 + 5rpm at 2250 lbs, sea level and 60 knots.
Rotor speed will decrease 10 rpm for every 100 lb reduction in gross weight and increase 6.5 rpm for each 1000 foot in DA (density altitude).

We started with autorotations at 60- 80 knots and full down collective, rpm around 523. The rate of descent was between 1500 fpm and 2000 fpm - ie fast . The we began varying the RPM.

We were at sea level and the temperature was around 20 degrees C. We could change the speed easily, but even raising the collective quite significantly kept the rpm around 490, it did not seem keep to go lower. However, at 490 rpm, assuming the VSI and altimeter were fairly accurate, we were losing less than 500 fpm, as we glided around and almost seemed to hover.
Changing the rpm, even from 520 (we cannot tell 3 rpm) to 490 (30 rpm) had a huge effect on the rate of descent.