This is a matter for golfers - those who talk about "feel"

Adding water to the get the right consistancy is about feel and experience really. You should be able to tell fairly quickly if its the right amount for th emanufacturer of paint.
However i strongly recommend the use of an acrylic flow improver. What you will notice if you use pure water and paint on a mini is that it goes blotchy and dries in all kinds of ways with no real control over the flow of paint. Adding a couple of drops from your brish of flow improver binds the diluted paint together and gives you fluid control. It then dries as a layer rather than a blotch.
Depending on your country depends on brands and availablility. In the UK going to a place like Hobby Craft and checking their paint range out will get you this product and is essential in my opinion!