It depends on the size and weight of the boat, and how much weight you are going to have in the boat.
You are supposed to be able to get to the manufacurers wide open RPM, with a normal load. But the
25 doesn't have a tach on it.
I would talk to a prop shop, there is one just East of Gene's Marine, or call a Mercury dealer, they know
how to prop a boat/motor combo.
You can also go to the Mercury or Quicksilver site, they have prop info. also.
They have some video's on how to do this also.
http://www.mercurymarine.com/propellers/prop-selector/