I have a tach and will use it to tune quite often. After I bring the engine to peak rpm for a given prop then I adjust the needle valve to an rpm 4 to 500 less than that peak as a setting. Most glow engines lean out in the air due to the ram air into the carburetor inlet in flight sort of a supercharging effect. Doing so prevents running the engine too lean often causing glow plug burnouts and/or dead sticks due to overheating.