Hey David,
Good observation because it is necessary for me to thin down all materials I spray with my airbrush.
Usually it is about 50/50 for me but it will differ for you depending on your air brush nozzle.
Shellac thins with denatured alcohol, lacquer with lacquer thinner, paint with whatever the base to the paint is. Acrylic is water, enamels are paint thinner, etc.
Looking forward to seeing your results!