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.

How does altitude get involved? According to the chart in the Lycoming
Operator's Manual, full throttle HP at Altitude there's roughly a
3% loss of HP for every 1000 ft. above sea level.