The first set of Turbo Dork paints was designed for use in an airbrush so the formulation was rather thin.
Then, as the paints became more popular and more paints were added to the line, the formulation was changed to better accommodate hand-brushing.
What happened, however, was that some paints ended up thicker than others. This was due to the unique mixture of pigments and medium plus additives that go into each colour.
When airbrushing, pretty much any acrylic-based thinner should work.
However, it is recommended that you use an airbrush medium to thin with. The Turbo Dork folks use Golden Airbrush Medium. Others have reported using flow improver as well to great effect, but we recommend using a flow improver in addition to an airbrush medium, not on its own. We do not recommend thinning with just water alone, at least for airbrushing.
At Exit 23 Games, we use Vallejo Airbrush Thinner and Flow Improver.
The bottom line is that no one set formula for diluting for airbrushing works for all colours.
For most colours, folks start with a ratio of 2 parts paint to 1 part medium. However, you will have to use your own judgement.
Some internet references say that the thickness of the paint should be the consistency of milk — you can go with that if it helps.
To be used for hand-brushing, paint is a little easier to accommodate.
Most colours are fine from the bottle. However, some are thicker than one might like and can be thinned with an airbrush medium, contrast medium, Lahmia medium, or by gently adding small amounts of water with a wet brush to the paint on a palette.