Proper mixture?

The general rule of thumb is to thin until you have the consistency of 2% milk. The amount of thinner varies from brand to brand, and even within a brand.

You just have to feel your way. Practicing on a primed milk jug will save your model from your mistakes, too.

Well I guess there are no hard and fast rules on the exact amount of thinner to paint ratio. Thanks for the reply. I'll have to get me a milk jug now.

I do an exact 50/50 mixture. It'll give you your milk consistency. Works well for me and easy to remember.

Andy

Great, I can really do 50/50. Being a Marine we try not to use complex ratios with big numbers. Haha.

I have to ditto Ross's advice.  I have had different bottles of identical colors of the same brand require different thinning ratios, so the concept of a hard and fast thinning ratio really doesn't work.  I have even had paint which was perfect for airbrushing straight from the bottle.  The main thing, particularly as you are getting used to what properly thinned paint looks like, is to practice on a non porous surface, hence the plastic milk jug suggestion, prior to painting on your model.  The reason for using a non-porous surface when testing is that if you spray on something like paper, the paper could easily absorb a lot of excess thinner that would pool up if sprayed on plastic.  A majority of your early airbrushing success or lack thereof will be directly attributable to getting your paint to the proper consistency.

Andy