I say do what you need to do! I would recommend moving up to 24mg though like KitKit said, maybe alternate between 18-24 for awhile, see how that goes.
I don't think there really is any such thing as too much vaping, especially coming fresh off of cigarettes. If you DO vape too much, your throat will dry out, you'll develop a headache... in other words, you'll have plenty of warning that it's too much, and can adjust accordingly.
You sound as though you're doing fine though. Don't worry about how much you consume until later. Now that you're not smoking, you've got lots of time
