Unless you have $$$$$ to spend, you will end up with a machine that gets you in the ballpark. I have a cheap F1 and it does just fine. For accurate fps for drop charts, you either pony up the big bucks or reverse calculate the actual muzzle velocity by shooting in known conditions (temp, pressure, humidity, altitude and distance). Most calculators can run the math for you.
The F1 I own is usually within 20-50 fps.