Unfortunately, the only way to find the right nic level is (ultimately) trial and error. Nic is delivered differently when vaping and is dependent on a myriad of factors. I'll try to explain some of the "science" behind it, so you can see what's going on.
It's pretty easy to calculate your estimated nic intake from traditional cigarettes. For example: I used to smoke 20 Marlboro Lights a day, so my nic intake was as follows:
20 cigarettes * 0.6mg nicotine per cigarette = 12mg nicotine per day
Even though the absorbed nicotine content of each cigarette is itself an estimate (based on research carried out by BT), 12mg per day what I was shooting for when I started vaping. What you need to understand, however, is that the absorption rate of nicotine from vaping is not taken into account when we are talking about e-liquids. The stated nicotine content of any given e-liquid is the total amount of nicotine per ml of liquid before it is vaporized. There have not been enough tests/studies to confirm the absorption rate of nicotine from e-liquid, but the best (current) estimate is around 30%.
Using 24mg e-liquid as an example, that means that only 7.2mg of nicotine is being absorbed for every ml of liquid vaped.
24mg * 30% absorption = 7.2mg
Someone like me (who was aiming for a 12mg daily intake), would have to vape 1.67ml of 24mg e-liquid to saisfy their cravings:
12mg/7.2mg per ml = 1.6ml
In reality, my vape pace equates to a lot more than 1.6ml per day - so I have to use a lower strength e-liquid. To give you an idea of what my vape pace is: I like to vape the way I smoked. That means 10-15 drags every hour or so. I have found that (vaping on a carto tank at 8 watts) means I go through about 4ml per day. To get to my desired intake of 12mg per day, I now use 12mg e-liquid:
12mg * 30% absorption * 4ml per day = 14.4mg nicotine per day.
Clear as mud, eh?! Bottom line: start at a "medium" level (e.g 18mg) and vape as much as you need to satisfy your cravings. If you find you're having to vape all the time, consider upping the nicotine content of your e-liquid. Believe, me: your relationship with nicotine will change very quickly once you give up smoking. Long haul flights, for instance, (which used to be absolute torture) are no problem at all for me these days!
