Being a maintenance test pilot, let me explain to you that one cannot adjust autorotative R-RPM.
If I had any number of maintenance manuals to hand I would post the procedures laid out for doing just that. I have done such flight checks on Bell 204, 205,206, 212, and 412's; Sikorsky S-58T and S-76's; Chinooks and Alouettes and MBB products.
How about explaining why one cannot do so please?