With a regulated mod the resistance of the coil is essentially irrelevant.
Regulated mods separate the input and output voltage, in other words they separate the battery from the atomizer. The only relevant value is the wattage, and the remaining voltage in the battery. As the voltage falls, the mod will increase the amp draw to maintain the selected wattage. You need to know the amp draw at full charge, and when the battery is discharged as this value will be the highest. Most regulated mods are about 90% efficient, so you will also need to factor this loss into your calculations as it will marginally increase the amount of amperage pulled from the battery.
To find the amp draw use I=P/V (-10%)
Eg.
75w divided by 4.2v equals 17.8 divided by 0.9 = 19.8 amps
75w divided by 3.2v equals 23.4 divided by 0.9 = 26 amps
With multi battery devices, simply calculate the amp draw then divide by the number of batteries.
If you are using a single battery mod and you go with either of the batteries Mooch suggested, I would stay under 60 watts to be on the safe side.