Try:
RPM x Pitch x OD x 60 / 5280= MPH with no slip.
Pitch is in feet (16" pitch is 1.33333')
OD is overdrive in box; 25% over is 1.25
60 converts minutes to hours,
divided by feet in a mile...
My Canyon:
16 pitch prop, 29% gears, 7500 RPM
7500 RPM x 1.3333 x 1.29 x 60 / 5280= 146.6 MPH
That combo typically nets me about 128 MPH or about 13% slip. My Canyon is pretty efficient, meaning it cleans up pretty well. Rides on the center pad and the chines dry. I've played with the numbers of some pretty fast flats and they're in that ball park. I'd guess realistic slip numbers for a hot lake boat (flat) in the 15%-20% range but YMMV.. Also, I've never messed with numbers for hydros, wackers or I/Os but the math is pretty much the same...
A couple other factors. Drag increases at the square of the speed. Power to overcome that drag goes up at roughly the cube of the speed (for the engineers out there, yes there's some simplification here, but it's close enough for the girls we go with

) Again and as always, YMMV....