I've always used the fact that I know a standard malrboro red contains about 1.2 mg of nicotine, so I consider 0.05mL of 24mg/mL juice to be equivalent to one cigarette. in other words, 1 mL of liquid to 1 pack of marlboro reds at the 24mg/mL strength. Then I go from there to decide how much nicotine I "need"
to the OP... alcohol in your eliquid can also add to TH in my experience