Doug wrote:
Rule of thumb. So if you are burning 10 gallons an hour in a 180 hp
engine, you are developing (10x12) = 120 hp. 120/180 = .67, so that is
67% power. It's not exact but gets you close.

Sure, but unless you have a fuel flow gauge, you don't know how much
fuel you are burning, and you would have to resort to rpm and density
altitude to figure out the % power.