It's not an easy question to answer as a lot of the data is still not back on hard rates of absorption, which will vary between users. Some data suggests an average cigarette has around 1mg of nicotine. From that the user may absorb 50% of that. So each cigarette may yield 0.5mg nicotine into the body. It should also be noted that a lot of cigarettes include nitrogenous bases to their blends to make the nicotine more of a freebase when heated which increases the bioavailability.
Now if we look at vaping, 1mL of 12mg liquid has 12mg of nicotine. It's hard to say how much of that we absorb, but it's not unreasonable to presume (for the sack of this breakdown) that it's the same as a cigarette. So we will presume 50%. This means that from one mL of that liquid consumed we've absorbed 6mg of nicotine.
If all this is "accurate" then in the process of vaping that 1mL of liquid we have consumed a rough equivalence of 12 cigarettes.
This may seem like a lot, but like I said the science is still not hard set and the math is fairly fluid. I am sure that a persons mucous membranes vary in absorption rates if they smoked for a year vs 10 years.
The best bet for gauging your dosing is to pick a moderate strength and feel it out over the course of a week and see where you fall.
In the end, if we are consuming more nicotine through vaping than smoking it is still better than smoking. Scientific studies have shown isolated nicotine to be no more harmful, physiologically, than caffeine.
