I've been mixing my own juice for a while, and recently was asked the reason for nicotine levels being numbers like 6,12,18,24 and so on. It got me wondering, because I use 100mg nicotine and make mine at 10,15,20mg simply because the math seems to be easier with a decimal system like milliliters. It just seems more complicated mathematically to me to use anything else unless, I suppose, you're mixing in bulk. If anyone could help me with this question, I would at least be able to answer the next person that asks me.
I have thought on this subject as well... knowing that in the beginning of vaping, back when, there were no studies to say how much nicotine was absorbed from the vapor the atomizers created, I had the thought that perhaps it was easiest to divide the 100mg nicotine concentrate into quarters and add PG/VG in the %s the makers chose... that gives you 4 groups of 25mg - and 24 & 25 can be divided pretty easily by 6, 5, 4, 3, and 2 giving you (easily) the levels they chose (6, 12, 24, 36). Perhaps they just halved the initial 100mg nicotine a couple times to come up with those levels, too - (and dropped the partial mls ?? lol)
I found when mixing up some batches in my goal of eliminating the nicotine from my e-juices, that I had no problem with a drop of 4 or 5mg in each level - of course it helps to become familiar with the e-juice calculators along the way - haha. Oft times when I'm in the market for some more nicotine for my DIYing, the 100mg is sold out, but they have 60mg - so I buy that because it too can be divided easily into different levels, ie: 12mg is 20% of the 60mg, so I just multiply my total juice mix (# of mls) by the 20% (Xml x .20) to get the # of mls of 60mg nicotine to add to the mix (to end up with 12mg nicotine).
NancyB
p.s. I found the first 2-3 days after dropping down to the next level of nicotine, I'd have a craving for a higher nic level - so I would not use up ALL my juice in one level before attempting the next level.. then I could take a few puffs off the higher level and be happy with that, and later in the day return to the lower level again - that saved me many times... a couple days down the road, I was no longer 'needing' that higher level. I was dropping about 4-5mgs to the next level in the process.