Clark never mix in the paint cup as paint goes around needle and that does not mix well with paint and thinner added above.  Always mix in another container - I use a large measuring spoon.  There are two Vallejo paint  types: Model Air ready for spraying which I find rare ( I have spayed only a few times with paint directly from the bottle) and Model Color for brushing that always must be thinned.  Before any thinning I remove eye dropper bottle top which does come out of bottle and stir paint inside bottle and reinsert eyedropper top.   thinning:  I mix all paint for the airbrush in a large measuring spoon.  Normally 30 to 40 drops of paint.  The beauty of the eye drop top.  Thinner: Model Air due to bottle sitting on shelf for a period of time may need to be thinned when opened so start with maybe 10 drops of thinner and work up 5 drops at a time. You should have a milk viscosity for spraying.  I use a piece of rail to stir the paint and thinner in the measuring spoon.  If I turn the rail after stirring vertical and the paint on the rail forms a drop and drips off paint is ready.  For Model Color I start with the same number of 30 to 40 drops of paint.  I start with half the number of drops, 15 or 20 of thinner and work up - again five drops at a time.  Again, when drop falls off rail used for mixing thinned paint ready for spraying.  On Model Color at times I will need and work up to the same amount of drops, 30 to 40 to get viscosity of thinner and paint correct.  Sounds like a lot of tedious time spent mixing; however, soon second nature and you will be able to mix at light speed as you building speed.

And, one key to Vallejo paint and thinner spray mix is needle size and air pressure.  The larger the needle size the better and air pressure 24 lbs or lower.  BTW when I started with water base paint I started with the Paasche H.   Now Passache Talon with #3 needle or Badger Patriot which has only one needle size.

Lester Breuer