Here's what I use in my equations. First, for your engine, determine the MAP that the factory shows on their chart of the power-altitude-rpm graph. That is the MAP that you ratio to with your engine's running MAP. For instance, on my O-235, rated power occurs at 28.4", not 29.92! So if I'm cruising along at 8000' and 22", the first part of the power ratio will be 77.5%. Next, you have to ratio your actual rpm vs rated rpm. So if rated is 2700 and you're at 2500, this part is 92.6%. This asumes that these engines have a fairly flat torque curve over a range of rpm so that power is directly proportional to rpm. So now we have 77.5% (0.775) times 92.6% (.926) which multiplied together gives 71.7%. Then to this I ratio the inlet temperature plus stagnation temperature rise and minus fuel evaporation drop and convert this to absolute temperature and divide this by the sea-level absolute temperature minus fuel evaporation drop, and get the square-root of this. So on this standard day the OAT was 30.5F and at 200 mph the stagnation rise was 7F. I add 30.5+7-24+459.7, 473.2, and divide this by 459.7+59-24, 494.7, giving 0.957, and a square-root of 0.978. Multiply this times the previous 71.7% and get 70.1% power Now without MAP and all of the temperatures, I could have just gotten the density ratio relative to sea-level for 8000' on a standard day which is 0.786. Some smart people found that on average, because of how the engine reacts to the induction temperature's effect on the inlet density and power, that if you raise the density ratio to the 1.135 power that will compensate. 0.786^1.135=0.761. Multiply that by the rpm ratio, 0.926, and get 70.4% power. But that only works for WOT, so if you have the throttle reduced, you need to use the first more-complete calculation with MAP. Nothing to it, right?