I would think you're not far wrong with that. You need to know how many revs your engine does at WOT (wide open throttle) and the corresponding speed. Your engine revs should be in the WOT operating range specified by the engine manufacturer.
For example, I get 42kts at 4800rpm at WOT. This leads me to think that my engine may be slightly over-propped, as it's supposed to reach about 5300rpm. However, I need to re-test again, as this was with 4 passengers, tents, luggage, full fuel load and an unbelieveably large ghetto blaster on board. Perhaps a lighter boat load will show me a better top speed!?!