It will depend on what your material is that your spraying and the viscosity of it. What kind of material are you spraying? Solvent or Acrylic?
For Floquil [solvent based paint] I thin it just a bit, then I'll use 20-25psi.
Most will say 18-30 psi.
I would say start with 18-20psi and adjust as needed. If the material looks as if it's spattering, not going on smooth and even, then increase the pressure a bit to help atomize it more. If this doesn't help, then try thinning the material a bit.
If your material looks rough, or looks like powder, the decrease the pressure a bit.