Can someone explain how the aiming works?

I've searched for a while and can't find any solid information. It seems AC3 uses a mix of aimed and auto-aim for projectiles with neither working 100% of the time. For instance, I am sneaking up on an enemy from a tree, I equip poison darts, press L1 to lock-on and fire but I miss. So I try manually aiming the reticle over the target and firing, however because it isn't locked on, the reticle moves freely as Conor throws the dart. This is the same for most projectiles (guns being slightly for reliable). Any advice would be appreciated as this problem is rendering ranged attacks useless.