Mh, here's something you might not have known... The 0.9mg on your cigarettes is sort of an estimate of how much you absorbed. Each cigarette could have 10mg or more nicotine in it but you only absorb about 1/10th of that so that's where your 0.9mg comes from.
The same is sort of true for ecigs. I've seen some studies (which are on the boards here somewhere...) showing about a 10%ish absportion rate again. The way it's absorbed is differently to real cigs too. It builds slowly rather than peaks like taking a real cig. So little and often seems to work for most for getting nic.
It's around 20 drops per ml but that's a pure guesstimate and could vary depending on bottle size etc. So for each 20 drops you vape you will be in contact with 1ml of x mg. Ie if your ejuice is 24mg strength it's 24mg per ml. Every 20 drops is 24mg you could have absorbed. But, if it's about a tenth of that you actually absorbed, then 2.4mg. Maybe two and a bit cigs for you?
Everyone is different, and if the research was right and my maths adds up it should be a good baseline to work off. Best way to know how much nic you're getting? Pay attention to your body. If you notice you're feeling something is a bit iffy, it's time to put the ecig down. If you're craving smokes all the time, have a session on the ecig.
Until there's solid research done on how much is actually absorbed we can all agree on, it's not an exact science and more how you feel you're doing.
Hope that makes any sense lol