Did you try my suggestion of mixing the two levels? It just might be (part of) your answer.
And I do understand... we all have our psychological thangs...
Also, I know quite a few vendors, and from what they've told me, 18mg. and 24mg. bottles outsell 6mg. and 12mg. bottles, so 24mg. is not necessarily a lot more than most e cig users are using.
If it were me, I would have cartos filled with both 18s and 24s...switch 'em up at a moments notice.