We use a pharmaceutical weight measurement as below:
E-Liquid strength measured in milligrams (weight) per milliliter (volume); expressed as mg/mL. Often the “per mL” (/mL) is dropped in conversation or informal writing.
Volume measurement such as “mix in the store” as below:
The amount of nicotine is reported as a percentage, milliliters or milligrams. When the nicotine amount in an e-liquid is expressed in either milliliters or milligrams by volume, it is usually implied that this is in a 100ml/mg measure of e-liquid. Therefore an e-liquid said to be 1.6mg in nicotine strength would contain 1.6% nicotine with the remaining 98.4% being made up from the other ingredients.
Hope this clears up some questions.