Your motor can only produce a given amount of RPM. But that RPM does not show you how much power you are getting from the motor.
You need a manifold pressure gauge to see how the motor is working. Let's not fly by RPM setting, let's fly by % of power. On takeoff with a FP prop it's simple.... full throttle check static rpm and go. The CS guys set their power to 75% by going to what they call 24 square. 2400 rpm and 24" mp. 24+24=48.
So now let's set our fp prop to 75% power..... Any combination of rpm (first 2 numbers) and mp that adds up to 48 will give you 75%.
And 65% is 46. 55% would be 44. As you climb, your mp will decrease. So to keep your power setting, you will need to increase your rpm.