I smoked 1.5 (occasionally 2) packs/day Pall Mall Orange (Lights) which I think had the same nicotine per cigarette as Marlboro Lights 100's - .9 per cigarette or 18mg per pack. Assuming I would vape as much as I smoked, I went with the per-pack amount of 18mg nic per ml and figure I am
vaping the equivalent of 1.5 packs. It's working out OK for me. I think 18mg is a fairly typical amount for lights ex-smokers.
When I started I also bought some 12mg in case the 18mg was too high if I overdid
vaping. But I only overdid once with some Peach ejuice that just didn't have an "ok - stop now" feel to it.
For the person above - I think Dipropyl Glycol is a different thing from Propylene Glycol (i.e. PG)