Karen, it may be helpful for you to know that
eliquid strength is measured as mg per ml. So your 18 mg liquid is 18 mg per ml (one exception to this is apparently Johnson Creek liquid, which uses the standard of mg per 1.2 ml, so their 18 mg liquid is somewhat lower in relative strength than everyone else's 18 mg liquid).
It has been estimated that approximately 20 to 25 drops will equal to 1 ml, but drop sizes will differ, so that is truly only an approximation. You could measure your own drops, but that would still differ from liquid to liquid depending on thickness, so you see it is really not necessary or even practicable to try to be so precise. And using carts as a measure of your usage is not too helpful, because they are all different sizes, and you rarely if ever use up all the liquid in a cart.
It's probably easiest, to have an idea of how much nicotine you are using, to simply keep track of how many ml per day you are using. For example, start out the day with a 5 or 10 ml bottle, and see how much you have used at the end of the day, or the week for that matter. The average use seems to be about 2 ml per day, with of course some people using far less and others far more.
But even knowing how much nicotine liquid you are using isn't going to tell you how much nicotine you are actually absorbing into your body. There is some preliminary evidence that we are all getting far less nicotine than might have been thought previously. See:
http://www.e-cigarette-forum.com/fo...5034-nicotine-absorbtion-vaping-research.html
Ultimately, you may enjoy your
ecig more if you just relax, and let your body be your guide as to nicotine intake. Really, you would know if you were getting too much, and equally if you are not getting enough.