Yes, it is trial and error. The screw at the rear moves the needle forward and back which in turn allows more paint to spray. If you need more paint screw the needle out, if you need less screw the needle in. The 200 is a single action airbrush so the trigger only controls the airflow.
Everything to do with an airbrush is effectively trial and error until you find out what works best for you. Air pressure, paint viscosity, needle position, distance from the model surface, etc. all have an affect on how the airbrush works. They take some practice (a LOT of practice to be able to do something well) so I'd also recommend that you use an old model to play on.