Wouldn't anything that affects horsepower also affect manifold pressure and rpm on a normally-aspirated engine? That suggests that the overall power will be affected, but not the percent power.
Example - leaning: as I lean the mixture, the power goes up, and then it drops. Fuel flow changes and so does airspeed. Since the engine can only make a certain amount of maximum horsepower, the percent power is changing along with the fuel consumption and airspeed.
Dave
If we say X engine puts out 180hp at 29.92 inches and 15C IAT at SL, with all other significant factors (many more than MAP and RPM) taken into account, we may call that 100% power. Actual hp varies with these multiple parameters and can also be expressed as a percentage. Overall (actual) power and % of our nominal maximum are therefore really the same thing. 90hp and 50% power for example in this case.
On leaning, it depends where you start from- richer than about 12.5 AFR and yes, you'd will gain power as you approach 12.5 and then start losing power leaner than that.
With a C/S prop, rpm changes are masked since the governor tries to maintain the set rpm by changing blade pitch. AT WOT assuming minimal restriction across the throttle plate and throat, there would be little resultant change in MAP since MAP should be close to the same whether the engine is running or not.
MAP does not change significantly with IAT, nor exhaust back pressure either.
I agree, fuel flow, hp and TAS are affected by leaning.
There are so many factors that influence HP and therefore % of power, that's it's virtually impossible to calculate % of power at altitude accurately without significant testing of that particular engine in an altitude dyno cell. Some of the factors are quite interrelated, such as ignition timing and AFR.
I wouldn't attempt to split hairs by 1-2% based on a simple formula which doesn't account for all influences. Your formula would get you in the ballpark however.
I would do all flight testing at WOT to minimize pumping loss effects in the data. The same goes for rpm and AFR.
In the end, the aircraft does what it does for TAS vs. FF at a given altitude and power level. That's really what matters and really your "dyno".
I'm a big fan of the F-4 Raider concept BTW but you really need a turbo on there to get back the knots lost with 2 less cylinders. No reason why a good 360 turbo can't exceed atmo 540 numbers above 10,000 feet or a little lower running a bit of boost. The price delta is significant these days and the lighter weight is also a big plus to get back some of the climb performance. If you're going to have a Rocket of sorts, make it one in performance too which is the main reason people want Rockets rather than RV8s IMO.