The needle regulates the amount of paint in the spray mixture,not the width of the spray pattern.
To spray a fine line move close to the subject,depress the trigger for air,gradually pull back on the trigger until your line starts to appear. Too much paint when close to the subject will flood the area. I lower the pressure as much as I can,but not below 12-15 psi,and thin the paint as much as I dare. I spray enamels not acrylics. I found subtlelty and acrylics were a poor mix for me but others manage okay.
With practice any typical
airbrush with a nozzle around 0.3 - 0.5mm will spray a fine line. I regularly do Luftwaffe style,feathered,camouflage demarcations freehand,even in 1/72 scale with a 0.3mm nozzle.
Steve