I've never had to dilute it for brush application, as it's pretty thin to begin with. The main thing is applying it relatively quickly, as it can dry somewhat fast. Having too little or too much on the brush is key as well. As far as diluting paints with it, it's not an exact process. What I do when I paint is that I have a reservoir for my Future (usually a tiny mixing up) and my paint in the bottle beside me. I usually don't need to dilute paint much, so just a tiny portion of Future will do the trick. I'll then mix the paint into the reservoir. I'll test the mix out on a sheet of paper to see if it's right or not. I just keep adding some of each until it's right for me.