hey kameleon! (Groningen area? Just a guess)
I use a 70-30 ratio for post shading, weathering or a wash. for anything else its way too thin. I use tamiya acryllics and am very happy to thin them to a consistency that thay just leave a transparant film of paint on the glass if swirled around in the jar. A lot of it comes down to practice really. Like with cooking, adding enough so it seems right.
Water is a bit tricky, and I've found that a nice big bottle of spiritus goes a long way and works very well. Not only can you thin the paints, but you can also clean your airbrush afterwards (or annoy your parents/family by using their Bols.)
if you need some help in your native tongue, send me an email at xj_mike@hotmail.com
Groetjes uit Zwanenburg..