OK, OK, I withdraw the formula---

 

Grabbing the 1985 Cessna 152 POH, assuming that MAP = atmospheric pressure at standard conditions and 2400 rpm:

 

alt   POH   Formula

       %HP   %HP

2000 74      76

4000 70      70

6000 67      64

10K  63      58

 

I am still looking for a way to chart % power without knowing the max sea level power that is delivered by the engine.  If I need different charts for different altitudes, so be it.