Thrust calculators assume that all propellors are created equal. The truth is some are more equal than others.
I'm interested in knowing how much my prop/engine combo can bench press without feeding the force through some Rube Goldberg setup. I set a stand up where the engine has to lift itself plus a known quantity of weight against gravity. I know what the ball park amount should be based on what a thrust calc says, then fine tune the test load with .5 oz weights until equalibrium is reached.