Uhh - the 'top black line or the bottom black line'. I do not understand what you mean by this, unless you are talking about the plunger of the syringe...
If you ARE on about the plunger then as you look at the syringe from the side (level with your eye, pointing straight up), where the top of the plunger is (assuming you've got no air in the syringe) is the amount of juice you've got for the shot. A trick for drawing from a vial (sealed with a rubber plug) is: say you have 2ml in the vial that you want to draw, pull back the plunger, filling the syringe with 2 ml air. Stick needle into vial, inject 2ml air. Now draw out the plunger (holding it up so you can see the gradations on the syringe) until you hit 2ml. Simple.
Haven't you ever seen this kind of stuff on TV? I know I have.