PC "However he does have a rate-of-descent curve for autorotation " that would do since it is what we are talking about and is the equivalent of Rate of Descent
I agree with PC that a Power Curve should give a reasonable approximation to the Power (and therefore rate of descent) required in autorotation, it might not be exactly the same, but probably good enough (as many recognised design writers also state)
Crab, i think using fuel flow would give a slightly unhelpful error in that the engine whilst producing no useful power might still have a fuel flow (in the region) of 60% of it's maximum. That would not translate into Rate of Descent in terms of the proportionality you mention.
I'm not buying the vortex ring state bit in autorotation, (really?), even if it is Prouty (or you are sure that is what he's saying?). Vortex perhaps, but VRing, I doubt.
MOSTAFA quite correct
also I found a published curve here FWIW