Yes, as Steve said there is no secret to thinning. I start with a 50/50 mix, or 1:1 if you prefer, of paint and thinner. Don't worry if it is not exact. Anything close to this will spray well, but get out a scrap body, or just some use plastic pop bottles, and experiment with thinner and thicker mixtures and see what happens. For experimentation purposes, don't be afraid to go to extremes. It's all a learning experience.
Also, your single action Paasche is capable of doing an excellent paint job. It is not necessary to go to a dual action airbrush.
Now, I am not the best to give advice. I have been painting (airbrushing) for decades and my paint generally sucks. I definately adhere to the method of; If you can't paint, get good at polishing. As such I won't give specific tips on painting technique lest I should steer you wrong, but I want to say that there are other parameters that effect paint job quality other then thinning. Important factors include air pressure, spraying distance from the model, speed at which you move the airbrush, the method in which the coats of paint are laid down (starting with mist coats, etc), and airbrush maintenance & condition. Experiment with these parameters, or at least seek/heed the advice that good painters suggest on these elements.
Youtube vids are a good place to start. There is a model paint DVD by Don Yost that many modellers claim has helped them to learn how to apply good paint (I have not seen it myself). And despite your success in mastering the airbrush, I will still encourage you to develope polishing skills for excellent results.