It's all trial and error, Jee. The more you use it, the better you will become- at least, that's the theory.
I use a Harder and Steenbeck brush with a 0.3mm needle (and smaller for fine detail), as this is a handy size for 1/16 tanks. Some are so precise, that they can be used on fingernails (not mine!). You could always practice on the wife
Ahem..well...practice on an old model, as has been suggested, or even spray onto some cardboard before beginning, just to check that the flow is even and manageable
"Get your facts first, and then you can distort them as much as you please"- Mark Twain.
????? from the 80s 
