As Elm says exact ideal ratios vary based on color due to pigment content and type, but generally speaking a 50/50 mix of ModelColor and either their thinner or water (or combination thereof) should produce an adequately fluid solution.  Of all the ModelColor shades I have there is only one I can recall ("intense blue") which needed a fair bit more thinning than that.

The correct mix will seem a tad bit heavy if you are used to solvent based paints but it WILL go on nice and fine (use at least 20psi).  It may also appear thick on the model to the newly initiated but these paints have fantastic shrinking qualities as they cure.  Come back in an hour and the coat will look finer than when you applied it.

I keep my water in a $0.99 bottle from Michale's which has an almost identical dropper to the one on the Vallejo bottles.  Makes it dead simple to get a given ratio (10 drops of water...10 drops paint....or whatever).