It depends on what kind of cigs you were smoking.
I rarely recommend 24mg to anyone, based on my own experience with that, as well as at least 10 new vapers I introduced vaping to. Basically, they either got woozy, sick, nauseous, but for me it was anxiety, a feeling of restlessness, and feeling very imbalanced in general.
If i do recommend 24, I always say get some 12mg in SAME flavor, cuz if you mix up a little ejuice, say 3ml, same amounts of 24mg + 12mg you get 18mg.
If this works, then you now have some 18mg to vape. BUT---if your "sample" of 18mg proves too much, now you have some 12mg juice to vape, without starting all over again.