The goodness of a rotor design for total power loss is determined fairly easily. It is a measure of the stored energy in the rotor as compared to the power the rotor is consuming at that instant. This term is important for auto entry, and also for the collective pitch pull at the bottom, both situations where the stored rotor energy is your life's blood. The energy ratio-ed to the power produces the time available for a given rotor rpm loss:
[Stored Energy] / [Power] = Time
[1/2 x I rotor x (omega) squared]/horsepower needed = seconds of rotor spin
Where "I rotor" is the polar moment of inertia for the rotor system (a term that is the resistance of the rotor to rotational acceleration, and which is mostly the weight, rotational rpm and radius of the blades.) Light short blades spinning slowly are bad, heavy long blades spinning fast are good.
For a "good" helicopter the calculation is about 2 seconds, for a poor rotor it is less than a second. Most Bell products have excellent numbers, the typical 206 is about 2.5 seconds, most Sikorsky's are moderate, about 1.7 seconds, and some of the very light composite systems are less than that, because the designers were told to make the blades very light, without considering this auto entry/landing as a design condition.