If you want to cut nic, try doing it in 1 - 2 mg steps, a step every couple weeks. Easy as a diy person, but even with commercial eliquid it is simple to mix batches at different mg, as long as the flavors match.
2 parts 18 & 1 part 12 = 16
Equal parts 18 & 12 = 15
1 part 18 & 2 parts 12 = 14
Start with a couple ounces each of 18 & 12, the above gives 3 intermediate mg to help you cut from 18 to 12.
(Sorry for the diversion, but being able to cut your mg is a good thing if you wish to, & there is no need to take big step sizes.)
2 parts 18 & 1 part 12 = 16
Equal parts 18 & 12 = 15
1 part 18 & 2 parts 12 = 14
Start with a couple ounces each of 18 & 12, the above gives 3 intermediate mg to help you cut from 18 to 12.
(Sorry for the diversion, but being able to cut your mg is a good thing if you wish to, & there is no need to take big step sizes.)