Basically it is this, the higher in watts you go, the more liquid you vaporize, more liquid used, more nicotine delivered. I explain it this way to break it down...
Pack a day smoker just getting started
Ego Class Battery (3.7v) and a basic clearomizer like an Evod (1.8ohm coil), 18mg nic liquid, doing around 7.5watts (1 to 1 ratio)
Doubling output a bit - iStick30w with a Nautilus at about 15watts, 9mg nic liquid would suffice, but a lot of people run 12mg so doing 24mg instead (2 to 1 ratio)
iStick50watt with a Kanger Subtank Mini, 0.5ohm coil at 30watts, 6mg nic will suffice, doing about 18 to 24mg nic after the math, (3 to 1 ratio)
Super sub-ohm dual coil dripper at 0.25ohm on a Sigelei 150watt mod at 60watts, 3mg nic liquid or less will suffice, 12mg nic roughly could even be as high as 27 to 30mg, could be higher the higher the wattage (4, 5, 6, even 7 to 1 ratio once you break down the math of how hot the person is running)