The reason to use a tach is to be able to set the engine slightly rich from max rpm. No one's ear is good enough to discern 200 rpm, especially with a 4-stroke. As was said before, starting from a rich needle setting, slowly lean the high speed needle while watching the tach. When the rpm has reached the max value, open the high speed needle a little so the rpm decreases by a few hundred rpm.

For safety, never rev a 4-stroke up while you or anyone else is standing in front of the prop. They like to throw props if the engine ever gets a little lean.