Hi, Mike.
There are no clear cut conversions here. The mg/ml does not translate cleanly into an
absorbed nicotine amount. That depends on a lot of factors, from the type of
vaping gear you are using and the power output to the humidity in the room, most likely.
However, data from NRT studies showed that people can reliably self-dose their nicotine intake, so let yourself find the right concentration that works for you. There are to sides to the nicotine concentration - on the one hand, it is likely that a higher concentration will allow faster / more nicotine absorption, but on the other hand a higher concentration also affects the 'throat hit'. You have to balance the two.
Many new vapers find that sometimes they need a stronger 'hit' to get them over those cravings, either due to triggers like meals, or morning coffee or just out of the blue. So a lot of us, at least in the beginning, use a lower mg
juice throughout the day, while keeping a higher one on hand as 'rescue' for cravings.
There is no right or wrong, however - what works for you is the best.
Rule of thumb, though - light smokers ~ 12-16 mg/ml; full flavor smokers ~18-20 mg/ml; hardcore chain smokers 24+ mg/ml...