As I was an ultra light smoker, I found 24mg to be WAY too strong! Because of the severity of the headaches and insomnia I was getting, I assumed that I had to bump down to 12mg and 18 for the strong cravings.
After trying the 12 and the 18, I found myself struggling with cravings more often. Eventually I had to order some 24mg for when I was out at work and only had 5 - 10 minutes to vape, or was drinking/really craving a smoke.
So, it seems to follow that the decrease in Nic level really is noticeable from 24 to 18 , from 18 to 12, etc. I now use a wide variety of nic levels and I've found that this works best for me. I always carry some 24mg with me, and usually a 12 and an 18. I find that I can match my Nic levels to the situation better this way while still fighting off cravings.
A typical rotation would be 24mg at work with limited time to vape, really need to have a satisfying quick vape here. At home I will usually vape 18mg if it isn't too late and it isn't a flavor that is impossible to put down (i.e. bubble gum, hopscotch, forest fruit). If i really am going to be vaping a lot, I've been vaping a lot prior to that point or it's getting close to bed time. I'll usually pop in a 12 or 6 mg juice. At these points I feel my craving is more vaping related than a need to replicate analogs and the lower nicotine has all but eliminated my headaches and insomnia.