Crab,
if I understood things properly, Nick and FOM between them have already answered that question. On the left hand side of the curve (left of the knee) we have established that the throttle is wide open so that we are not capable of producing our 124hp. If we come from an ISA situation to an ISA +10 situation we have dramatically improved the operating conditions for the engine as the engine benefits much more from the lower pressure altitude than it suffers from the higher temperatures (thermodynamic boffin stuff). This increased engine performance has the effect of 'cushioning' the effects of a change in density altitude so that we pay a price of 50 feet for every degree of deviation from our standard day rather than 120 feet. As Nick points out it's not just a question of rotor performance, you also have to take the engine into account. This cushioning effect disappears to the right of the knee where engine performance becomes constant below the critical altitude producing a steady 124hp. Now we see that for every degree of deviation from our standard day there will be a loss/gain of 120 feet. It becomes purely an aerodynamic issue (more or less). At least, that's my understanding of the issue. Continued enlightenment always welcome however.
It's a nice day, and the sea looks inviting. I think I'll go for a swim!
Irlandés