It is very true what's been said above but I'm going to wade in with my

(can we get an English option for that?!).
Even if you absorbed 20mg of nicotine (hypothetically) in smoking 20 cigarettes back when you smoked, and absorbed 2ml of 18mg juice now when vaping, don't stress out too much about this. Tobacco, as has been said above, contains so many other chemicals which get you more hooked, more quickly. You could still have major cravings as you come off the stinkies.
If you're getting a rapid heart rate, feeling dizzy or nauseous- let's just say that if you start to feel ill - then lower your nic level. Otherwise, enjoy not being on the stinkies and get used to that before worrying about dropping your nic too far and too fast.
As zapped said, lowering your nic level too quickly can be a total waste of time. I started on 24mg in the mornings, 18mg all day about six months ago and now I'm sticking at 8mg for a bit because I know I'll go loopy if I drop any quicker. I average (since Februrary first) 1.65ml juice a day.x