29.4 ml
Basically, put in 3 bottles.
18mg x 29.4 ml = 529.2 mg of nic.
Add that to 500ml and you have a final volume of 529.4 ml of juice, containing an extra 529.2 mg of nic.
529.2 / 529.4 = 0.999 extra mg per ml.
Can you really notice the difference between 3mg and 4mg?
I can on my first vape of the day or if I haven't vaped for a couple of hours.
Once my nic levels are up it's all the same.