Of course mixing the 24mg and the 18mg 1:1 will get you 21mg. My experience is similar - 24mg was too much but I could vape 18-20 without the same effects but even the 18 was satisfying for me. So I could see where a mere 3mg difference could work for you.
Another factor is nicotine absorption and while it would vary with each individual - you know your range, so the more vapor produced - better volume on surface area of the lungs to absorb - a set up that creates more vapor - either more voltage or a low resistance atomizer on what you're using already, might allow you to get better absorption on the 18mg ejuice.
Again, in my own experience, I was in pacifier mode (power
vaping) when I had the regular 510 kit which runs at @3.2volts and has an atomizer that runs at @2.1-2.3Ω or about 4.5 watts. It is only when I got a box mod that ran at a true 3.7volts - with the 510 atty - @6.5 watts - to where I could put the thing down for a while. I was getting more vapor and better absorption using the same mg nicotine ejuice.
Another ploy that I used was 'vape low - top off high' - where you, eg. vape at 18mg normally and then when the urge really hits - do a refill with the 24mg. That way you don't build up a tolerance for the higher nic level and eventually you can do the same thing with say, 12mg as 'low' and 16 or 18 as 'high'. The way I got off cigarettes was by deciding to hit the ecig
first for three hits and then If I wanted a cig after that I'd have one. When the nicotine from the ecig hit the bloodstream, the urge went away.
