I have a 15 mil bottle of ejuice with zero nic. I also have a bottle of 18 mg of nic. 50 pg/vg. I want to add the 18 mg of nic liquid to the zero nic eliquid to make it 6 mg. How much of the 18 mg nic do I need to add to the 15 ml of zero nic??? I hated chemistry in school and it is coming back to haunt me now.