There are lots of theories about how to reduce paint, with some artists promoting specific ratios, and others telling us to reduce paint to the consistency of milk.
The problem is that ratios don't always work the same way for different types of paint and air pressure settings, and reducing to the consistency of milk isn't always appropriate either. In the end, mixing by "feel" is just guesswork.
I propose another way of doing it.
I don't usually reduce my paint, since the Createx Illustration line that I use is designed to be sprayed right out of the bottle. However, if I'm doing intricate details, I sometimes reduce my paint slightly with a 3.5 to 1, paint to reducer ratio.
The next goal is to get the air pressure optimized with the paint reduction. You will want the maximum pigment load, but with the best flow, so you can easily make fine black lines. Too much pressure will cause the paint to "blow out" and splatter.
The next step is simple: I start with around 30 psi, then gradually lower the pressure until the atomization of the airbrush begins to fail. Then I turn up the pressure slightly above that failure point. I want the pressure to be just above the threshold of failure, so that the pressure is as low as possible and still maintain atomization.
In this way, you will have maximum pigment load and proper atomization.
That's it! Best of luck in your airbrushing pursuits.