New airbrushbrush rookie

The Paasche 2000H is an external mix, siphon feed airbrush. The 30 psi air pressure is a good starting point. However, it also depend on the accuracy of your compressor pressure setting. If you get a misty spray and the paint looks smooth and even when it dries, you are doing well. If the compressor pressure is lower than what you think it is, you may get splatter or uneven spray. Aaron Skinner has a good basic airbrushing article in the latest FSM.

The 50-50 thinning ratio is a good starting point too. It is a compromise on how fast (how many repeated passes) you need to get a solid color vs. the final surface finish. Try thinner paint if you feel the dried paint is too rough.

Good luck and enjoy your new tool.

The Paasche 2000H is a updated version of the old venerable H airbrush.  It will spray fine at 20 PSI with both enamels and acrylics.  You could probably go a little lower on the pressure, just see how the paint atomizes, and adjust your air pressure appropriately. 

I also use MM paint (and even the little square bottles of enamel), and generally start with 1:1, but it depends a lot, especially how old the paint is. I start just a bit shy of 1:1 for brand new stuff, increasing thinner with number of times bottle has been opened.  THOROUGH mixing is important with Testors enamels.

I personally think 30 psi too high. I do flats at 20 psi, gloss at 15.  You need to be closer to the surface with gloss paints to prevent too much drying before paint hits surface, in order to get a good gloss.