what paint ratio works for you?

In my experience, there's no "one size fits all" magic number for paint thinning.

Paint viscosity can vary between different colours within a particular manufacturer's range and even different production batches of the same colour. It can also vary depending on the age of the particular bottle of paint you're using. Brush-paint out of the jar for an hour and it may change due to evaporation of the carrier from the jar.

General "rule of thumb" suggests thinning to the viscosity of milk. However, just how much thinning you require will also vary depending on your equipment setup, the effect you're trying to achieve (thin lines vs. broad coverage) and your own individual painting style.

For example, my personal preference is to build up colour depth in mutliple layers rather than try to achieve instant full coverage in one coat. Where many people suggest two parts paint to one part thinner for Tamiya acrylics, I often use anything between two to four parts thinner to one part paint (and not measured with any sort of real accuracy) and then build colour depth using multiple passes. This is my own personal preference and it suits the way I paint but it will vary from one person to another.

I would suggest thinning to a milk-like consistency and considering that your AB's are siphon feed, start around the 18-20PSI mark and experiment a little.

Basically, ditto Phil.  There is simply too much inconsistency between different bottles of paint, even when considering the same brand or even the exact same color of the same brand to be able to come up with a meaningful thinning ratio.  After a little experimentation, you get the hang of it.

As far as air pressure goes, I have a Badger 200NH which I use for enamel painting and a Paasche H which I use for acrylic painting.  I typically paint a 15-20 PSI range.  When I'm laying down an overall base coat with my airbrush further off the model, I'll be in the upper end of the pressure range.  If I'm doing detail work, like freehand camo with my airbrush close to the model, I'll be in the lower end of the range.