My compressor runs shy of 30psi, but w/ the airbrush open, it drops to ~17psi. My air tank can be run past the 50psi limit of the hose, but I don't see why I would want to run it greater than 30 psi while open. So I'd say ~20 will be the typical usage.
But this opens up another question: why one would vary the pressure so greatly in the first place? I've heard people run their brushes from 15-35psi. I've been stuck at 17psi for the last couple years so never had the ability to experiment until now'ish.
I vary the pressure a lot myself. Lower pressure can spray thinner paint without puddling. Higher pressure can spray thicker paint without clogging or stippling. Then there's stippling (sp?) Nice effects can be achieved with thicker paint and lower pressure, like stains, mud splash, grease splash, etc.
Also lower pressure can let you get up closer to the work without puddling. I do this when I spray subtle stains on small items that will be top coated. Best example I can think of right now of what I mean is painting panels that are stained all round the edges. I coat it with the finish color. Then I get in as close as I can with black around the edges. Then I go over the whole piece with a thin coat of top color. This mutes the stains and gives a nice effect on mechanical items, like Gundams for instance, as if they've been in service and have been serviced.
Also, if the area to be painted is small, you can avoid masking a large area by lowering the pressure and getting in close enough without fear of overspray.
Mostly though I adjust the pressure up and down to suit the paint I've just poured in the bottle. I don't really measure anything, I just go by eye and experience. I test a little spray, and adjust pressure as needed.
Pressure adjustment can be done quite easily with a regulator and pressure gauge inline with your hose. I use a tank, so its: tank, on/off valve, drier unit, regulator, pressure gauge, hose. But I bet you knew that already.
Hope that helps.