Hmm its hard to say...
I have a bottle of water with flow aid mixed in (think it was 20:1, but I followed the flow aid bottle guidelines-ish)
When painting I bring an amount of paint out of a pot onto a pallete, and also put an amount of clean water/flow aid in another well on the pallete. then move water into the colour just bit by bit with a brush untill satisfied with the viscosity.
How thin? well as thins as possible without detracting from the coverage too much. Experiment and practise is the best advice I can give... You want the paint to be thin, but not as if its a wash... too much would be counter productive and its rather hard to describe. Just try it out and play around with how thin you can take it before you feel its working against you. And remember its okay to put one coat on and it to dry patchy then apply a 2nd thin coat and it will dry smooth (or atleast less patchy each pass) Its very much a less is more kind of mentality, you want the minimum amount of paint to provide the colour you desire and no more