When you vape 24mg it's 24mg per milliliter. So a 4ml tank is going to give you the heated version of that amount of nic in the juice, some of which is actually absorbed through the mucous membranes and the lungs and the rest is exhaled.
It's not a direct correlation to the amount of nic in the juice. Smoking a cigarette containing the same nicotine level is going to deliver blood levels much higher than the vapor.
Nic is transported into the lungs on ash particles that make up the smoke. Those ash particles stick to the lining in the lungs and are absorbed through the alveoli (the air sacs in the lungs), and to some extent by the mucous membranes in the mouth and throat.
Studies have shown that we get 1/3 to 1/4 the nic from vaping that we get from tobacco cigarettes. The good part is that we get enough nic to satisfy the urges and allow us to transition from smoking to vaping with much less trouble than most other nic delivery methods.
Add to that the fact that we still retain the hand to mouth actions of vaping and our urges are satisfied, even as we transition from high nic to low nic or even no nic. Part of the addiction is the action of smoking that gives us something to do with our hands and vaping replicates that action.
Most of us don't realize that vaping has lowered our nic needs to a level where we can drop the nic concentration to low levels with little or no cravings. I found that out when I started dropping my nic in steps until I was down from 24mg to 3mg. I went from 24 to 18 to 12 to 6 in about 6 weeks. It was really easy and I went through almost no cravings at all.
3mg is all I need and I can deal with zero, but I miss the throat hit at 0, so I vape 3mg to get a little of that bitter flavor.
Nicotine Absorption when Vaping - Mt Baker Vapor