Sorry to nit-pick, but on a regulated device it's the wattage you choose that drives how long the battery lasts.
In some of these Boost/Buck converters, they have higher efficiency when they operate near the input voltage and less than it ((battery) 3.7-4.2). To maximize battery life, I aim for the dead voltage, 3.2 Volts. Therefore if I like to vape my Subtank Plus at 31 Watts, I make a coil that will run at 31 watts with 3.2 Volts. That maximizes conversion efficiency. Otherwise you are just making heat inside the mod. PWM + cap or switching with cap is cheap in power loss to drop the voltage (buck), boosting the voltage always lacks in efficiency.
(For single or parallel units only)
.5 ohm coil should be fine up to around 20 watts.
.4 ohm = 25W
(my coil is in the middle here)
.3 ohm = 35W
.2 ohm = 50W
Watts = V^2 / R, so R = (V^2)/W = (3.2^2)/Watts = 10.24/Watts
10.24/31 = .33 ohms, which I use in my device (single 18650). You'll notice the longer battery life at same
wattage.
I haven't seen any difference in run time with dual 18650's in series, as I never build over 1 ohm to begin with, and I don't vape at 64 watts. (I use a mech for that stuff)
One caveat... If your unit boosts the voltage to 10V (from 4) or something, then bucks it down to whatever it's set at, I doubt this would have any effect.
What kinda difference? Probably 95% efficient near the battery voltage, dropping to 80% if it's way above the battery voltage. That's just typical boost/converters from a datasheet.