What is the powerplant you're running in the boat? The rule of thumb should be to first look at the Max RPM of the engine, not the top speed. The max RPM is usually in a range. Then get the boat at WOT, trimmed properly and see what you're actually getting for Max. RPM. you can play with raising tabs up and trim up until you can max. the RPM. Compare that to the manufacturers published max rpm range. If you're within the Max. RPM range you're good. If you exceede the max rpm range, you're under propped and if you're under the max. rpm range, you're overpropped.
Personally, I would try to work with a local prop shop. If you explain what you're trying to do, they will usually lend you a couple of sizes to try, then yo ucan buy teh one that works best. There are also prop calculators out on the mercruiser sight which should help narrow things down as well.
http://www.mercurymarine.com/propell...-choose/pitch/
http://www.mercurymarine.com/propellers/prop-selector/#
I know I didn't directly address your quesstion, but I hope this helps.