Research I've read about says the smoke from one cigarette has 1mg of nic.
depending on the cig brand and strength. Some are only 0.8 and some are 0.5 or less, whcih is actually substantial when trying to advise a new vaper about what strength eliquid they should be starting at.
Formula is loosely (i.e. will put you into the ballpark of what nic level a newbie should start at): 1 pack a day = 20 x 0.8 (in my 0.8* example above) = 16 so 16mg would be a good place for them to start.
(*you have to look up your brand in the tobacco/cig brand database to get that 0.8 number which corrersponded to my brand)
I was seriously over-nicked when I started here, (despite the fact that vaping delivers less nic than a cig) becuase people on the forum just gave me a blanket strength "you smoked a pack a day, you need 18-24mg eliquid".
Of course, I didn't KNOW why I was 1) sick, 2) jittery, 3) anxious until some kind person PMed me here and figured out what I should have been vaping. Everyone kept telling me these were symptoms of "quitting cigarettes"......... BUT IT WASN'T!!!!
Turns out I was supposed to be doing 12mg or less. AS SOON as I lowered my nic I was comfortable again and it is what allowed me to vape full time and quit cigs.