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 adding different levels of juices, you want to use this equation:
(mg.#1 x ml #1) + (mg.#2 x ml #2) / (ml#1 + ml #2) = Total mg
(18mg x 10ml) + (18mg x 10ml) / (10ml + 10ml) =
180 + 180 / 20 = 18mg
20 = 18mg (So you are left with 20ml of 18mg).
Here is an example with different amounts of
juice that has different amounts of nicotine.
I have 5ml of 18mg nicotine, and 10ml of 6mg nicotine. If I mix them together, what nicotine strength liquid would I be left with?
(18mg x 5ml) + (6mg x 10ml) / (5ml +10ml) = Total mg
90 + 60 / 15 = Total mg
150 / 15 = 10mg
juice
15ml of 10mg juice would be left.