Each propeller has a specific load/power curve.
The good engine testers had calibrated "props" that acted as air brakes. After a lot of juggling with atmospheric pressure and relative humidity, wielding slide rules, they came up with test power curves, almost as good as sophisticated dyno data.
Taking simplifaction one step further, use the calculator I develloped (not all my work!)
http://www.mvvs.nl/prop-power-calculator.xls