I find I absolutely need 24mg. I only smoked regular Marlb menthol's and was almost a PAD, but anything below 24mg requires me to chain-vape solidly for 15-20 mins before I feel even remotely satisfied (which is really inconvenient, especially if I am on a limited time table, like in the morning rushed to get out the door on time).
The only drawback to 24mg or higher is that it can be a bit harsh, I often get the 'cheese grater' exhale with such a high nic level (especially first thing in the morning or if I haven't vaped in a long while). But I get my nic satisfaction much faster (5-10 minuets), which is similar to my analogs (which 100% satisfied me in 5 mins or less).
You could start high or you could start low, both have the advantages. If you start high, you may get used to the higher nic level and it will end up being a requirement for you (sometimes can be hard to find 24mg or higher). However if you start too low, you may end up feeling unsatisfied after vaping for long periods of time and may lose interest int he whole e-cig thing cause it can't combat your analogs efficiency.
I say you buy a low, medium and high to start. Start low, but if you need more you can always try the higher nic one. And if the low works for you, you can always dilute the higher nic level ones to a lower level (can't do that with lower nics levels to make them 'higher').
Also don't forget that vaping is a much less efficient way to get nic into your bloodstream than analogs. Some may think 24mg is borderline dangerously high, but it's still not nearly as potent as an analog puff for puff (in terms of how much your body receives).
Just depends on your situation. If you just want to social/recreational vape, which would mean longer periods of vaping to enjoy the flavor, then a lower nic level is the way to go. But if you want something to curb the nic craving as quickly as possible (as analogs are so good at doing), higher is the way to go.