topendtorque (and others)
If anyone tries to mandate a 10% power margin, ask them to show you how this is going to be accomplished using the performance charts in the flight manual.
Unless you have access to a military flight manual for exactly the same helicopter type, the civil charts will not give you any way to calculate power required to hover.
You simply can't get there with a civil performance chart. (If you can, please let me know how you did it!).
Power available is different - that might be available from the power check charts, but that only works for a specification engine.