Just to help with the mg/ml thing:
1 kilogram = 1 liter
1 gram = 1 milliliter
1000 milligrams = 1 milliliter
So for example, 72 mg/ml PG means that 72 out of 1000 milligrams is nicotine, and the remaining 928 milligrams are PG - hence the nicotine concentration in this solution is 7.2% in this example
(this is nicotine concentrate remember, you would never vape this, a typical
ejuice will be around 1.2% nicotine which is 12 mg/ml or normally referred to as just 12mg)
If I want a 8 mg juice (or more precisely, 8 mg/ml), then 1/9th of my ejuice would consist of the solution mentioned in the above example.
This juice would have 8 mg/ml of nicotine, and 992 mg/ml of everything else (PG, VG, flavourings) - this would be a 0.8% nicotine ejuice.
If you know this then apologies, just though I would post this as a little understanding helped me be confident that I wasn't mixing juices with dangerous nicotine levels