What PSI setting are you using ?

As you said, many variables. I find the majority of my spraying is 14-20 psi, no set formula, I just mix up my paint, then adjust the pressure until I reach the "sweet spot." Often I might just ignore the pressure gauge, making my paint thin-ness/pressure adjustments by sight and feel. You just kind of know when you "reach it."

I'm with Patrick on this one. I have a MAC valve on the end of my AB hose and the regulator on the compressor set to 25 PSI. I adjust the MAC valve to get the spray I want and don't look at the pressure gauge - you just sorta know when you reach the pressure you need without actually knowing the pressure. In use, I close the MAC valve, paint in cup, and open MAC valve slowly until I get the spray I want. Works for me and all I know is the pressure is less than 25 PSI.

I generally apply gloss at 15, may go up to 20 for flats.

I'd have to agree with patrick. Anywhere withing 14-20 depending on the type of work I'm doing.

Dash8

I know it varies alot but I have found 20 psi

is the magic setting for my BD-130 airbrush

using MM acrylic paints thinned with water

like 2% milk, I get a very nice fine spray finish

at that setting using a .3mm needle.

 

I never, ever use water to thin Model Master acrylics when airbrushing. I use Testors universal acrylic thinner instead. Adheres paint better than water would.

Black Sheep raises a very valid point, even though some paints work well with multiple thinning agents, others are more demanding and will work poorly if not thinned with the best suited ingredient.

One example, more than one acrylic can be thinned with water, alcohol, lacquer reducer, windex, maybe some additional I don't know of. But try using alcohol with Vallejo Model Air, you'll become a real expert at airbrush cleaning. It will make the worst congealed mess you have ever seen. 

Best to investigate how a combination will work, (or not,) before commitment to using it in application.

Patrick