The mg rating is how many mgs there are in 1ml.
I'm a little lost by "I usually smoke 6mg." Is that per cigarette or how much you use in a day? If it's per cig, I would try going to 12mg. I say this because we don't know how much nic is actually absorbed into our systems. I've heard 30%, 50%, and so on but nothing has ever been verified by testing. I smoked 4 PAD when I quit and used 36mg juice. 2 months later, I started feeling a little ill so I dropped to 24mg. 2 months after that, 18mg which is where I've been since.
Nic absorption is different in each individual. If you're having terrible cravings, step up and try 12mg...then listen to your body. If you don't start having cravings, headaches, dizziness, upset tummy...then that's where you need to be. Stepping down can come later.