Basically it all depends on the individual. For example, I used to smoke 35 ultra lights per day (.5 mg per cigarette) but I vape about 5ml at 15mg and I feel great. Theoretically, at that rate, I should only be using about 7mg (since common held knowledge is that you only absorb about 50% of the nic that you vape). Anything less and I get cravings. Anything more and I start to get lightheaded, headaches, etc. Long story short, your body will tell you what will work for you.