Power is a linear function of the amount of air processed through the
engine. The amount of air is a function of the product of manifold
pressure and RPM.

For my Arrow, takeoff numbers 30" 2700 RPM, product 81000, 200 hp.

At 5000 ft, 25", 2400 RPM, product 60000. 60/81 = 74% power, 150 hp.

If I reduce to 2100 RPM, product is 52500. 52.5/81 = 65% power, 130 hp