Well, it's just a formula to calculate the relationships between voltage and resistance and power output (wattage) I think (*think*) that you'd just use the straight up formula, THEN take the voltage drop off at the end to find the true wattage you're vaping. I think that the resistance will affect the voltage drop, so you'd figure it just using the regular formula, then take the voltage drop off at the end. It think you're just asking when to subtract the voltage loss...??? I don't know......I feel really out of my depths here.....the blind leading the blind. Ha. Wouldn't you get the same number either way?