That is the problem== Different brand props do act different. Darn.The problem with props can be you get the RPM you are looking for and loose the performance you were trying to get at the same time.
Max horsepower is when the engine can’t spin the prop faster than max rated RPM. It has to be loaded. RPM and max manifold pressure is at that point. Of course at sea level.
Fixed pitch aerobatic airplane you would want 2700 rpm in vertical and you don’t care where it runs in level; you are not supposed to be level in aerobatics.
Top speed you would want 2700 in level.
Everything else is in between on how you load it.
My guess in the black magic is don’t go all the way to 3”. 1 1/2 or 2”. You don’t want to step over and turn it into wall art. It might be short on what you want but you also don’t want to go backwards. Different brand props do act different.
It’s good to see the data points with your findings.