The mg of nic depends person to person. Some people have issues determining their strength. I smoked 2 packs a day and ended up at 24mg which works good. A friend of mine also 2 pack a day habbit had to go to 16mg for his sweet spot, 24mg was to much. Best thing I can tell you, if you have a flavor you really like, get multiple strengths ie 12mg, 18mg, and 24mg and just see which works best for you personally.
I forgot the other important part is to find a go to flavor, I had bought 6 flavors when I started and purposly stayed away from tobacco flavors (but thats me). I was a menthol smoker and found Fire and Ice flavor a good go to. When I had the urge to light up I would switch carto to the fire and ice and it worked much better than any other of my flavors to get rid of the craving.