Maddafinga basically covered it. I too have found that it seems better to err on the side of two much thinner than not enough. I would also emphasize the importance of doing test spray on scrap plastic prior to painting on your model. Sometimes you'll have a paint mixture that looks perfect, but doesn't spray well. At that point you need to adjust the mix ration until it sprays right, regardless of it seems in the color cup or mixing jar.
On my most recent project, a MK-I Spitfire, I thinned out some ModelMaster Duck Egg Blue for the underside. It was fairly thin out of the bottle, and I thought that I had seriously over-thinned it to the point where I added a bit more paint to it. So I was quite surprised to find that the thinned paint was barely passing though the airbrush and laying down kind of lumpy in a manner consistent with underthinned paint. I added a bit more thinner and tested, and had to repeat the process several times until I was spraying something that looked more like colored water than properly thinned enamel. In spite of the apparent over-thinning of the paint, it went on beautifully and covered well.