Not sure about exactly which airbrush you have, but if the thing is spraying and there's no paint build up on it, it should be fine. Mineral spirits is fine for enamel. If you recycle your cleanining thinner, a can of Kleen Strip mineral spirits form Home Depot will last years.
There are not hard rules for paint/thinner ratios. Even within the same brand and color, you will find bottle to bottle variations. Usually 3:1 or 2:1 is good place to start. The usual rule of thumb is to try an get it to the consistecy of 2% milk. After a bit of practice, you'll get the hang of it. Do test spraying before spraying on your model, and always test spray on something non-porous like a plastic milk jug so that the paint will react as it would when being applied to the model.