It really depends. I would get a variety of strengths to try, from 10 - 36 mg. As was said, you can always dilute a juice that is too high. You cannot gauge what level nic should be in your juice by what you smoked. The absorption is different than an analog (mostly oral and throat, not lungs), so how much nic you need will depend on the absorption efficiency and your own body's metabolism. Try some juices and see what you like.
And to make it even more uncertain to predict, how much you vape in the beginning is not necessarily the level you will be vaping in a month. It might be more, it might be less. How you vape also important in nic absorption, and how often you hit. But a variety and just take some time, vape, enjoy, and figure it out as you go. There are no rules, no equations to follow.