15>20psi is a sensible place to start, but there are so many variables (type & model of airbrush, paint, thinner, personal style...) thats it virtually impossible to say that X or Y psi is the idea pressure.....
If the paint isn't atomizing properly, your pressure could be too low.
If you find your paint is going on rough or grainy, it could be an indication of excessive pressure drying the paint too much before it hits the subject.
That being said over or under thinning can also cause these problems - best bet is to play & practice (preferably on scrap) until you get results that you're satisfied with.