Rich,
Unfortunately, there's no sure-fire method outside of experimenting.
If the paint "spiders", then it's too thin, if it splatters, then it's either too thick, or you don't have sufficient air pressure coming from your compressor. I assume you have the ability to control the psi.
I don't spray enamels anymore (wife has bad asthma), but the consistency of the mix should be like milk if you're spraying between 25-30 psi. Higher pressure, you can go a little thinner, but understand you'll need more coats, and there's always the risk that the paint will dry too fast, in some instances, even before it reaches the model.
Also, when you spray, make sure you practice on a surface that's similar to the plastic of your model. Spraying on cardboard (unless it's the glossy coated variety) will give you different results than your model will.
Fine lines can be achieved with the proper mix and lower air pressures.
It takes some practice to nail this down with a dual-action airbrush.
Jeff