As a general rule, however much paint you use, use double the amount of thinner. I.e for 3ml of thinner, add 1.5ml of paint. I used small disposable syringes to measure out and drop paint/thinner into the airbrush cup.
I’m not sure if it makes a difference, but for me I always put the thinner in first. I’ve had better results that way, instead of putting the paint in first and letting un-thinned paint seep into the nozzle and glob it up.