Some data from my R44-I scientific simulator (quite a while since I used it..)
First the autorotation
at 80% rpm, fully loaded 60 knt auto
8-curve shows the angle of attack at a certain radius versus the max angle, based on a 4th order viscous flow stall model. 8-curve should remain below the stall curve.
AoA show the effective angle of attack of the blade segments
So at 80% : acceptable stall zone
Now at 70% rpm, fully loaded, 60 knt auto
so at 70% : fully stalled retreating blade
In between at at 75%
so at 75% : very questionable
Still 93% of retreating blade is stalled
Secondly the flair
Don't have any figures yet, but my guess is that at 75% you will run out of collective. So not so much stall, but collective limited
Hope that helps, convinces me not to go below 80%. Of course different TOW etc may be considered to refine margins etc, but I'll keep the 80% in mind as a pilot.
d3
(edited to get img links correct)