Go out at a weight that you know won't need take off power in an OGE hover, stabilise and check the actual MAP. Then see what rate of climb an extra inch of MAP - or 2 - gives you, it just allows you to calibrate your brain to how much or little performance you have left when you are pushing the limits of the aircraft. If you know you are only just below the max gross for an OGE hover then you will know you have to be super-careful if you have to go into a tricky confined area.
Done and past done, demonstrated and in every day use. I.e. the aforementioned 2" MP margin for max. performance and 6-8" for OGE hover. Rules of thumb, or brain calibrations if you prefer. Those guidelines work. No need to reinvent the wheel. Assuming a healthy engine, of course, which was the other major point you are making, I think.