I checked my tach recently against another brand and it matched right up on a .60 we were tuning, so I know mine is fairly accurate. As for what prop to use, that depends on what you are trying to do. For example my Stinger 120. It has a thick fully symetrical airfoil and is designed for slow speed aerobatics. That means I want to spin the largest diameter, lowest pitch prop that loads the engine to the point where it is in the correct RPM range. A 16-10 on the other hand is starting to get into high pitch speed/low thrust area. That's good for planes that want to go fast.

Jim