I often use Gunze lacquers, and so I usually mix about equally parts of thinner and paint. That would mean a thin but smoother finish. When used with Gunze thinner the paint will remain in good condition even after mixed with the thinner, so even for big projects I fill up only about half of the paint cup. I use a gravity-fed airbrush so filling it up all the way could result in spilled paint.
For acrylics and enamels the amount of thinner or spirits I use is less, but since experience tells me that the paint may begin to deteriorate after mixing I prepare an empty bottle where I can store the paint/thinner mix and re-use the same paint later for touch-ups.
I never thin the entire content of the bottle since, if I find the final mixture to be too thin I'm stumped, and will have to go to the hobby store for a fresh bottle.
Hope this helps.