When you refer to "optimum" setting, what do you mean? Have you leaned the engine to peak RPM? There is no actual value for the number of turns on the needle valve. You have to find it by seeing how the engine is running. As the engine is run, you can gradually lean it until you reach peak RPM. Two tanks' worth of fuel is not enough to break in your engine. We'd run the engine over a 45-60 minute period and gradually work the high-speed needle towards peak RPM. When the engine can sustain running just rich of peak RPM without much change in the mixture, we'd consider it ready to fly and fairly-well broken-in.