Yeah, I started at 18mg a day and at the time it seemed to feel like it was enough, but I was taking entire mouthfuls of vapor to get the hit I needed, and even then I was still vaping very frequently -- probably 4-5ml a day. I've since switched to 24mg and it's just about perfect for me. I don't take such great mouthfuls (maybe 3/4 a mouthful), I get a wonderful throat hit, and I don't chain vape as much, going through about 3ml a day. That turned out to be the answer for me.
I also started out on an eGo, which is what I've been using the whole time (and a 510 with a LR cartos when I want something light to take out on a quick jaunt) so that would also make a difference. Greensmokes are generally overpriced and not very good; even a straight 510 kit would be much cheaper than GreenSmoke and work much better for you, especially if you get yourself a pack of nice Boge cartomizers. You might even find that 18mg works better for you on better hardware -- but honestly, for a 2PAD smoker I'd go for the 24mg. I was a 1PAD smoker of regulars and even I prefer 24mg.