I agree with Kent that in general nic levels of juices are pretty close to where they are stated. That said, however,
vaping is NOT like smoking in terms of nic absorption. Smoking has ash particles that carry the nic directly to to lungs, so fast spike in blood levels, and also a relatively fast dropoff as the nic is metabolized. Vaping, OTOH, has nic in droplets of PG or VG "vapor", with the majority of nic being absorbed orally or in the throat or nasal passages. Very little nic actually reaches the lungs. So the absorption is slower since it is primarily oral/throat, and the dropoff is also slower, since nic is still absorbing as it is being metabolized. If I cannot vape or smoke or snus, I do find that the time before I am feeling nic withdrawal is significantly longer than with just smoking, by at least a couple-fold.
So while 18 mg is a good middle-road level for nic, and might be great for you, you will probably find that vaping and smoking are different enough physiologically that you will just sort of gravitate to a particular level, regardless of how similar you think it is to smoking. Personally I like 15 mg most of the time, but sometimes I want more, sometimes less. Even the way you vape changes the amount you get. Hard to hold your breath after taking an analog drag, since the CO makes you very dizzy. Not the case with vaping at all. You get all the nic without a problem. If you are fascinated with the vapor you are exhaling, however, and want to produce clouds and clouds, well, that's nic you are not getting.
IMHO, you should vape the level you like, and not be too concerned about how close it is to smoking. Apples and oranges, smoking and vaping are.