Either 24 mg/ml or 18 mg/ml of the same flavor can be used, especially if you actually want to nudge the nic down. Equal parts of 18 and 12 will get you 15 mg/ml. Two parts 18 and 1 part 12 will get you 16 mg/ml. (In other words, take an empty bottle, fill it half way with 18, fill it the rest of the way with 12, and shake -- you now have 15 mg/ml).
100 mg/ml is the highest strength nicotine base used in do-it-yourself (DIY). While it's not like plutonium dust, if you aren't into DIY you are probably best sticking with just mixing the 12 mg/ml with some higher strength of the same flavor.