The problem with rules of thumb is that the turbine engine is dependent more on temperature for power available than on density altitude (the explanation is too long for this forum, check out my book - the Art and Science of Flying Helicopters - shameless plug).
The example above proves the point - a 10C change in temperature made the engine temperature limited instead of transmission torque limited.
Rules of thumb I've seen worked for low density altitude conditions, and were related to the margin between maximum N1 or torque or TOT and the existing N1, torque and TOT in a three foot hover - these are good for the pre-takeoff condition, but of little help if you have to land at higher altitudes.
The favored way to look at the pre-land conditions is to go VY (typically 60 knots) and look at the margin between existing power and maximum (and you may have to pull to the maximum to find out which parameter it is) and then make your decision on the margin.
Sorry no hard and fast numbers here as they will differ from machine type to type.