Well everyone have different needs so there's not really a 1 size fits all solution unfortunately (some might stop smoking with e-cig or cigalikes) i failed with these devices and 15 mg is just the right point for me to get a satisfying dose of nicotine without vaping for longer times. I am thinking about sub ohm at the moment for those tasty clouds and because i want to start reducing the dosage as i'm getting more and more used to vaping + the consistency of the clouds i feel seemed better. I'm also in canada and i think 18mg is the highest dose of nic you can get here .Enjoy!
I'm not so sure about shops these days and what they recommend.
18mg in the old days was about the normal and certainly not only for heavy smokers. 6-8 watts was about the power then, of course.
I never bought the heavy smoker thing anyway back then; the logic (to me) seems that you smoked, you got your nicotine, you smoked again - sooner if you were smoking a lot. No particular reason to vape super-high nicotine levels.
The nicotine from one vaping session needed to feel similar (I guess) to smoking 1 cigarette.
Some folk used 24mg or even 36mg - those days must be gone now - even a modest e-cig makes twice as much vapour as the early eGo types.
One shop gave my friend 6mg and she was still smoking a year later. Too little is as bad a recommendation as too much.
Imho no solution is perfect unless it stops you from using analogs which is the biggest point of vaping.