Since vaping doesn't equate directly to smoking, finding the right nic strength can be a trick. My wife and I used to smoke the same cigs, about the same number of them per day. She vapes 6mg, I vape 24mg. We started at 12mg. It was too much for her, and it left me wanting. Apparently, I used to hit the cigs more often and harder than she did.
Once you do get it settled tho, the mistakes along the way can be remedied - for example if my 24mg had been too much, I could mix it with some 6mg and the result would be 15mg. So all is not lost - in fact, you end up with more juice out of the deal - certainly not a bad thing, right?