A quick estimate of what boost adds to a naturally aspirated engine. Pressure ratio does most of the work, and an efficiency allowance accounts for heat and drive losses.
Boost packs more air into each cylinder. At sea level, 14.7 psi of boost doubles the air pressure, so an engine would ideally make about twice its naturally aspirated power. Real engines fall short because compressed air gets hot and less dense, and superchargers take power to drive.
The efficiency allowance is a rough correction for those losses, not a measured value. At altitude, atmospheric pressure is lower, so the same gauge boost produces a higher pressure ratio but the naturally aspirated baseline is also lower.