It depends what you're doing when you say fine line stuff. You mentioned a shadow which to be honest you don't need a fine line for.
The needle and nozzle combo's you have for your badger should be marked in some way so as to show what they are, maybe F for fine and M for Medium ?
I have a couple of Iwata's, the HP-B is too fine for Faskolor, has a 0.23mm tip, then I have a medium one on a bigger brush which is a 0.35, this is okay for Faskolor but you'd probably get a better flow and therefore more control out of a 0.5 tip ! So to save buying yourself a new airbrush you could look around for a different tip set up for the one you have ?
Shadows and the like are all about control, you just need to let the airbrush do it for you though, so as long as you can pull a steady consistent line you should be okay whatever size tip you have unless it's clogging ? Practice Practice Practice
Try thinning the paint down a touch ( Auto Air Reducer works well ), also I use the window tint colour for shadows, it's thinner and is transparent. You can turn the pressure down more so you can go slower and control your line better.
Good luck
