I'd say you are well within the range of normal and could probably go 2-3 ml more or less and still be well within that range. The experience can vary significantly from one person to another. As for you still having a nic-craving, that could be for one or a variety of reasons. It is possible you would find a juice with a higher concentration of nicotine (36mg, for example) more satisfying, but it is also possible that simply modifying your vaping technique a little bit might do the trick.
I mention the latter because we must understand that we don't really absorb nicotine from vaper so much the same way as we do from cigarettes. With vaping, the nic is bonded more strongly to the suspension medium (PG and VG) than it is in cigarette smoke and as such, we don't actually get all that much nic delivered to our bloodstream via our lungs the way we would with a cigarette although the actual vaping experience is otherwise very similar to cigarette smoking.
The way we mostly get nicotine from vaping is actually more closely related to the way we would get it when smoking a pipe. Pipe smokers normally inhale some smoke, but very litte compared to cigarette smokers. Pipe smoking is all about the mouth, the nose and flavor. The nicotine in pipe tobacco smoke is very well absorbed into the bloodstream via saliva and the fine tissues of the mouthy (like with Snus) and with the fine, moist tissues of the nasal cavity (like with nasal snuff).
This doesn't mean one can't get the same satisfaction from vaping one can from a cigarette. It just means we get much of our nic a little differently than the way we do from cigs and that our nic-blood levels rise a bit more slowly than they would with cigarettes. However, I would mention that I think a person could probably achieve significantly higher nicotine levels in the blood with vaping than with cigarettes simply because the vast majority of people could vape much more than they could smoke. The result would be similar to the very high levels of nicotine in the blood that many users of nasal snuff (I love the stuff) and Snus users achieve (I love that stuff too).
Another factor we have to deal when moving from cigs to vaping that IMO is just as significant as nicotine blood levels is what do we do about all of the smoking-associated conventions and rituals we largely lose when we move to vaping? The number of things we do that we've somehow come to associate with cigarette smoking is mind-boggling. So, we get a big mod with a big tank and fill it with some juice we love. Then what? There's no buying/making cigarettes, there's no lighting or putting out, there's not the same marking of time before, during and after life events and for crying out loud, what do we do with all this extra time we have...AND THESE HANDS?
It might also be possible that you are a person who is craving, not just nicotine, but additional tobacco alkaloids that simply aren't present in the vast majority of e-juices. Their absence doesn't seem to bother most people, but it appears that it does bother some. If it turns out to be the case that this is what is bothering you, then you can either pick up a can of Snus or European nasal snuff (none of the American versions of either, but the Euro versions are hands-down the safest tobacco products on the planet) or find some info around these forums on tobacco alkaloids that can be added to e-juice.
Most of all I'd say, give yourself some time to figure out what it is that YOU need to do in order to adjust to vaping and get the most out of it...which is distinct of course from what the rest of us may need to do. I hope find some of this long-winded post helpful, herky.