Someone might have numbers based on actual data, but in lieu of that, using a reasonable approximation might be reasonable(sic).  A first approximation is that power is proportional to both engine speed (RPM) and manifold pressure.  Compared to the rated hp (HPr) the actual hp would be:  HPa = HPr * RPMa/RPMr * MAPa/MAPr.  But, as someone famous once said, "to tell the truth that's not quite true."  Engine friction hp subtracts from the indicated power (the power developed within the cylinders).  When close to full power ( I assume you only care when running something over 50% power) an improvement over the above calculation might be to add a 10% additional correction.  The correction works in the opposite direction for rpm as it does for MAP.  For a 10% drop in rpm from rated, drop the power by 9%.  For a 10% drop in MAP lower the power by 11%.  I could come up with an equation, but it's early and my brain cells are kind of slow :-)  For power settings below 50 or 60% this approximation rapidly falls apart, but do you care to know the power below that?  I doubt it.  And a bigger effect is due to air/fuel mixture while the above approximation is only good for a fixed mixture.  Takeoff power (full rich) probably drops the power about 2% from LBT (Leanest mixture for Best Torque) and running LOP will drop power by 5 to 15%.  So the above calculation might work ok for rich mixtures, but isn't very good when running LOP.  As George would say, LOP operation will produce power pretty much proportional to fuel flow and is independent of MAP and RPM(he has a number for this that I forgot).

Gary