Thinning paint is, in many respects, a matter of personal preference. How thin the paint needs to be depends on how much pressure you use and that, in most cases, depends on how far from the model you spray. If you spray close, at low pressure, the paint needs to be thinner or it won't spray properly. If you use a bunch of pressure (not recommended!) you can spray it thick. Find a pressure / distance / thinning ratio that you like and stick with it.
I usually spray at about 10 - 15 psi from about 4 - 6" from the surface, and I thin my paint about 3:1 (paint : thinner). If I am doing fine lines I'll reduce the pressure to around 5 - 8 psi, thin the paint more, and spray from about an inch away.
As Tho said, Tamiya paints work much better when used with their thinner. I normally thin Tamiya acrylics with 91% isopropyl alcohol, but that will make even the gloss paints dry flat.