I use a Paasche VL, and there's not really an optimal pressure setting to use, but rather, I vary the pressure, based on the job.  I do tend to brush generally around 20 psi, too.

I use a Craftsman 1.5 hp compressor with a 3 gal tank, too.  Multi-use--I can use it to paint, and there are also air-driven tools it can power.