Trying to dial in a coil ohmage that doesn't drain the battery so damn fast.
Techie Stuff
We all know that regulated devices work on watts-in-watts-out. Given 100% chipset efficiency (wishful thinking), the resistance of your coil will have no effect on battery life at any given wattage. The iSticks' chipset efficiency has never been published either, that I've seen. I low-ball a guess at 90%. But that's an average. A chipset's efficiency varies throughout it's operating range. It has been my experience that a (buck-boost) chipset's setting with highest efficiency lies somewhere around it's battery's native voltage. In the case of the iSticks, that's 3.7V.
Building your coil(s) at a resistance that allows you to obtain you desired vape in the 3.5-3.9V range should provide you with maximum battery life, especially with Li-Polys. There is a slightly larger discussion to be had regarding the iStick (30 &50) switching between PWM & DC-DC somewhere around 3.1V. A little experimentation should find your sweet spot. I build my RTAs & Clearos to vape between 3.7-4.5V. This keeps me squarely in the DC-DC output, which I prefer. I continue to get better battery life with my 30watter than my DNA 30 devices (w/ 2500mAh, 20A 18650).
Good Luck!