I'm new to acrylics, having used enamels (Humbrol) for the last 46 years. But, I've started using Vallejo Model Colour, and thinned it, with water, 50/50. I have 'messed about' with thinning ratios, and it worked reasonably well at 60% water, for a thin misting. Overalll though, 50/50 is better.
The good thing is, you actually get twice as much paint for your money when thinning for airbrush use, AND you can use it for brush-painting too.