I could be wrong, but I've read many papers that stated in a cigarette approximately 1 drop of pure nicotine could be extracted and that 1 drop was enough to boil blood as well as stop your heart, at almost 100mg of nic in that 1 drop. Add in combustion of the tobacco roughly you burn away 52 to 48% of that nicotine, so roughly smoking a full cigarette you get about 48 to 52mg nic from that cigarette, full PAD of smokes, 20X48=960mg of nic a day being exposed to in a single day as an example. Cig nicotine is also, cut down in the manufacturing process, from 15 nano-meters to about 7 to 8 nano-meters so it hits the brain and is absorbed more quickly and easier, the nic we are exposed to is its natural size of 15nm, thus is harder to absorb, myself in my Kayfuns I run 12mg and use about 4 mL a day that way, 4X12=48mg daily exposure, in my sub-tanks I run 6mg at higher wattages than my Kayfuns, averaging about 8 to 9 mL used a day, 8X6=again about 48mg exposure a day, I started out at 48mg on cig-a-likes like NJoys and later moved to a 510 cig-a-like kit, each carto was about 1 mL, and I'd have to burn through or refill those cartos at about 10 reloads a day, 10mL X 48mg=480mg nic exposure a day.
As you stated all in the math, but just myself since I started 5yrs ago, down to 10% what I first started at, which is a big win in my book.