I really am not brave on that, At first I used to be afraid of heights (wind etc..) But I studied things as retreating blade stall margins quite extensively on my simulator and know the margins in many circumstances.
Power margins on the R44-II seem to be very good, rotor seems to be more the problem ip retreating blade stall. For the R44-I, it was always the motor that quit first, even at FL85 and before VNE.
I built this up very progressively on many flights in many circumstances. Mostly now I would cruise between FL75 and FL85 because I feel its the best in many circumstances. From FL105 rotor limitations limit effective TAS, and consumption goes up so you are starting to loose again (not to speak of slimmer margins).
AT 12000, in a local mountain flight I stopped not because the machine couldn't do more (2POB, 2/3 fuel), but because the manual indicates NO FLY at any weight.
d3