knots are somewhat irrelevant. Heading into wind, waves, current, or tide will much different than running with them. At the same RPM with a 3 knot tide, you could be running at 20 knots heading into the tide, or 26 knots running with it.
I shoot for an RPM window. If that translates to a speed that is too high for the sea state, then I slow down or attempt to modify my angle of attack to soften the ride. More often the speed is dictated by the sea state.
Determine what the max rpm of the engine is, then subtract between 10 - 25% for targetted RPM range. You're amx speed will be dictated by RPM and sea state.
Trying to use speed at the metric for measurement is a bit like the tail wagging the dog.