Here are the real world rules of thumbs I paint by:
-Paint with the lowest pressure possible. Reduce the pressure until just before the airbrush stops spraying smoothly- thats the pressure you want to spray at.
-Use as much thinner/reducer as you can before the paint runs/pools. Thin it until just before it pools (and spray it on in light mist coats to avoid pooling)- this is the amount of thinner you want to use.
These can be judged only by test spraying- you MUST test spray. There are far to many variables in airbrushing (ambient temperature, humidity, operator skill, phase of the moon, etc) to expect that thinning by a ratio will work. You must find your own pressure and your own level of thinning by testing and experience.
...and that pressure will probably be between 20 and 30 psi for Tamiya acrylics, and you might have decent luck with oneish part thinner to twoish parts paint. Or so. But test it (always test it) and find out for yourself.