wesp:
Unless otherwise specified, the grey area in the HV diagram is for level flight. Don't assume an engine failure during a high powered at the edge of the 'safe' area will be easy to handle.
Wildwilly:
There is no formula for what you seek. The approximations that are typically made for helicopter performance will quickly fall apart as you transition from powered flight towards autorotation.
The answer for the required intervention time problem comes from flight test. If it's calculated for a climb, first a minimum Nr will be decided. Next, the time between an engine failure and pilot reaction will be increased until the extrapolation looks like a further increase in delay will reduce Nr below the minimum.
Nick mentioned an engine failure in a climb is demonstrated for certification. Check the applicable FARS/JARS/etc. to see what the requirement is if you need more details.
Matthew.