Most full strength cigarettes contain 1.2 mg of nicotine per cigarette or more. I think lights are usually .7-.9, and ultralights can be down like .5
The mg on ejuice is per ml
Some common starting ejuice nicotine ranges
(for the higher ends of range - some need more to start out, some just need that much)
Full strength smoker - 24mg, may need 30+
Lights smoker - 16-18mg may need 24
Ultralights smoker - 10-12mg may need 16-18
But it really depends on how you smoked, what PV you get (some vaporize more ejuice per puff because they have bigger attys), how you intend to vape.
For me, with my PV (which is a small atty), I chose ejuice nicotine equivalent to one pack of the cigarettes I was smoking and planned to vape the same amount (1.5 packs worth of puffs). If you search on Google for your cigarette brand and the word nicotine you can probably find how much nicotine per cigarette. Mine were .9 which x 20 is 18mg and 18mg ejuice worked out for me.
I did buy a group of nicotine levels as recommended for starting out. Like this (I was a lights smoker):
12mg - bought a small bottle in case 18mg was too strong or for if I vaped more than I smoked
18mg - bought a larger bottle, this is what I thought I needed
24mg - bought a small bottle in case I needed it certain times of day or all the time
As it ended up, I only needed the 18mg but I'll be using the 12mg eventually because I plan to cut my nicotine a bit. When I started out I was using about 1-1.5 ml of ejuice per day. Now I am vaping more and using about 2ml per day so I plan to cut down to 12mg slowly by using up some of my 18 mg while starting to use 12mg for some of my heavier vaping.
In the end it depends on you, how much you vape, how you feel. if you get a headache or rapid heart then you are probably using too much nicotine. If you are still getting cravings then it can be a variety of things like too low nicotine, a juice that doesn't satisfy and relax with its aroma and taste, a juice that makes you feel like you ate, not enough TH - it really depends on the person. For me, it was some TH and a comforting aroma like tobacco or cocoa for the "you just smoked" feeling, along with the nicotine.