I smoked 1.5 packs of Camel Blue 99s per day. These cigarettes are .8 mg nicotine per cigarette and are one step stronger than ultra lights. I found 2-3 ml of 24 mg e-liquid per day to be a good match for me.
Ultra light cigarettes are usually listed as .4 to .5 mg per cigarette. 2-3 ml per day of 12 to 18 mg e-liquid sounds about right if you were smoking 1.5 packs per day of ultra lights. I'd start at the high end of the nicotine scale (or vape more e-liquid per day) and cut down a couple of weeks later.