While I understand that everyone is different, these "accepted" generalizations for
vaping and smoking equivalency don't hold up for me at all. I've come to the realization that in the two weeks I've been
vaping, I've overdosed on nicotine several times...at only 12mg levels.
I was smoking ultra-light cigarettes, with .4mg tar each. While that number sounds very light, it was actually even less once I started thinking about it. For one thing, very few people actually smoke an entire cigarette; for me, 2/3 of it's length was more common. Second, of that 2/3 length only a portion of that was inhaled, since I usually stood around standing and talking more than smoking.

. Finally, what did get inhaled was usually about 10 somewhat shallow drags passed through a filter.
So with all that said, my actual nicotine intake was probably much lower than what it would seem at first glance. A little conservative number crunching puts it closer to .15 mg per cigarette. Assuming I was smoking a little less than a pack per day depending on how much I slept, I came up with about 6mg of nicotine inhaled in a 48-hour period on average, through 400 drags. (10 drags per cigarette... 40 cigarettes * .15mg each = 6mg).
For a comparison, I monitored my
vaping over a 48 hour period, and ended up at about 400 drags (coincidence?) and 1ml of 12mg juice used. So I was vaping about the same as far as drags and the physical aspect, but the over-simplified math shows that I was taking in twice the amount of nicotine.
My body's reaction supported that theory. I had headaches and a general feeling of something just not being right. Not to mention, after the first few days I was "needing" to vape more often than I "needed" to go have a smoke. I believe that was my body starting to react to the higher nicotine levels.
Since I now mix my own juice, I'm cutting to 6mg or less; I may even try 3mg and see how that works.