ahhh you hit the nail on the head Bob.. it's the orange juice bottle... it has to be a MILK jug.... ![Wink [;)]](/FSM/CS/emoticons/icon_smile_wink.gif)
For a brush like that, working for fine lines I would try hitting around 10psi to start, very little paint in the cup and start with about a 1:1 thinner to paint ratio... you can adjust the pressure as you see fit... you'll want to be in real close to the plastic, about 1/2 inch or so.
You might try doing the opposite of what I started with... push down on the trigger, pull back until you get a fair supply of paint and start moving the brush... move the trigger forward till it becomes a thin line and try to hold it at that width... this will help you get a feel for about where the needle should sit when spraying that combo. Once you can get it down, try the opposite way, push down and begin pulling back till it "feels right" Especially with acrylics this might be the best way to go. (heavy then move to fine lines until you can switch back to the other way.. There's not as much tip dry with this thinning ratio but it will help keep it down by starting heavy at first...
Oh.. and make sure you have that proverbial q-tip moistened with thinner handy... I'd go ahead and clean the tip after each line to help prevent tip dry whether it keeps on spraying or not.