If the data plugged into the formula is accurate , the math doesn't lie. This number is the absolute maximum the boat could run with that particular pitch prop and engine gear ratio. But there are no guarantees, nothing written in stone. Only real-world testing will establish optimum setup. For larger, higher-horsepower boats, a foot throttle is a necessity and wheel or floor mounted trim controls are strongly recommended.
Environmental conditions play a significant role.
Temperature and humidity can affect engine performance by several hundred RPM, typically 2-300 lower for a 100 degree day than for a 50 degree day. An area with little or no other boating activity and sufficient straight line running room with good visibility is required for safe testing. Testing should be done with a typical load of fuel, tackle and people. The objective is to determine the best the boat can do by controlling as many variables as possible. Even when optimum setup is achieved, day to day conditions will usually prevent using full potential.