I use between 25-35 PSI depending on what paint I'm using. Metallic paints I spray at higher PSI, they just come out better that way, don't know why. I use both Gunze and Tamiya acrylics. I used to use mixing pippets for mixing paints in spare bottles I had to get them ready for airbrushing. Now I just take a full pippet of Tamiya acrylic thinning solution and squirt it right into the paint bottle it came in. turns out that gives me a 3:1 mixing ratio works great for me.
And I now use nothing but the color cup mine came with, so much easier to clean and much less waste. I use a Paasche Millenium with a fine tip for everything I do.