Gene, Jim, Homebrewer,

Unloading is not a constant and Gene, you are right, it is a 'trial and error' afair.
You do have to take into account (predict) the amount of unloading, from the sleeknes of the plane (a Clark-Y trainer's engine will most likely unload much less than a QM pylon racer.

Also, the higher the prop pitch, the less it will unload at any given speed. With a low pitch prop; 5, or 6 and a sport engine, many sport planes may reach the point of "pitch speed".
Any further increase in speed; diving, the prop will become a turbine and spin even faster. But this cannot happen, unless your plane is a very aerodynamic, pylon racer, pseudo-jet, or the like,

With an 8, or 9 inch pitch prop, you will be lucky to reach "pitch speed", even if your plane is a pylon racer. But when these speeds are reached, the unloaded engine will spin significantly faster than the static numbers, even 3k more is not theoretical. This, because it was heavily laden with excessive pitch, on the ground.

So if you are into QM, or Q500... do give it more needle on the ground.

Sincerely,