The missing part of the equation is that the absorption rate of nicotine is not 99% like it is with a cigarette. They make them that way on purpose.
The truth of nicotine absorption when using a pv is somewhere between 10 and 40 percent. Lets just use 40% for the absolutely worst case scenario. Personally I believe the truth lies in between the two extreme numbers.
If I smoked a pack a day at 1.4 mg. Thats 28 mg of nicotine.
4ml of juice at 18mg at 40% is 28.8 mg.
Thats quite a lot of vaping at the worst case scenario. That being said, I let my body tell me when I've had enough nicotine, or not enough as you will just know. It is really hard unless you get into the highest mg levels, and heavy vaping to actually use more nicotine than when you smoked and even there I have my doubts. I'd honestly expect people that use very high nicotine levels, and who are heavy vapers to also have been heavy smokers. In either case, I can tell you that I feel so much healthier after the switch, there is no doubt in my mind, the pv is by far the better alternative.