If you are adding the same amount of different juices with different strength's or even the same (let's say 10ml of 18mg, 10ml of 24mg, and 10ml of 6mg), then you just add the nicotine levels, and divide by the number of juices. 18+24+6=48, 48/3=16. In your case 18+18=36, 36/2=18.
If you are...