How much nic you are getting is actually not a straightforward assessment. Blood tests have shown that given the amount of nic vaped, we end up with 10-40% of that nic remaining as nic in the body, with much of the rest showing up as cotamine, the oxide metabolite of nic. This is likely because much of the nic we vape is not taken in through the lungs, but orally and in the throat. Analogs delivery far more directly to the lungs, so the nic levels systemically are much higher.
So I have given up in trying to accurately figure out how much nic I am getting compared to smoking, although after taking into account these numbers it looks to be roughly the same in the end. That said, I simply vape until I am satiated, but the amount and concentration of the juice can vary depending on circumstances. The range of absorption amounts also is rather wide, and likely depends on the person and the way they hit, making the whole comparison thing, at least to me, rather moot.
That certainly doesn't keep companies from using some bogus comparisons to assign # analogs to carts, but this is just done to rope desperate smokers into their little scam. If you see these silly comparisons on some ecig website, run!
How much you get from a cart also depends on the quality of wicking and atty health. I use the ZFM mod on my 510, and whereas before with the stock cart and filler I would need to vape 24-36 mg to feel right, now it is 12-16 mg. Juice thickness also relates to this, and much of this difference I see is because I thin my VG juices some with vodka or water now. Too many variables to really compare well. Make sure everything is running optimally, and then just vape till you are satiated.