[chris vickery]

Stuart, if you are using enamel paint just go to the auto parts store and purchase some medium reducer to thin your paint.
I always go by eye when thinning but I have done a lot of spraying.
The auto parts store should be able to help you on the correct amount of thinner. They do sell a stick which you dip and then count drips per minute to acquire the correct vicosity.
I believe I start around 10 to 20% reducer mix.
Other types of thinners will affect your results. Typically things like toluene or xylene will flash off quick and dull the finish. Depend on your gloss level or lack thereof...