I'm with the others about just using wattage instead of the voltage setting.. it's easier to deal with.
Ohm's Law isn't really that difficult... what makes it seem difficult is that you have 4 potential values you are working with... so there are several different formulas and which one you use... depends on what value you are trying to calculate. Each formula by themselves are not that tough, basic simple math.... Still, it's easier just to use an ohms law calculator to do the calcs for ya. Here is a little schooling on it anyway just for the info...
http://www.hamuniverse.com/ohmslaw.html
Mod limitations... While the mod in question is able to output up to 70w... that is still dependent on it also not exceeding any of the other limitations. There are the minimum/maximum voltages that must not be exceeded. The ohms ranges also must not be exceeded. The final limitation will be the total amperage that the device is capable of. This is one place that being able to calculate ohms law can help you avoid exceeding the specs of the device.
When you listed the ohms resistance of the coil ( at 0.4Ω ) ... you asked where it'd get burned out at. The only problem is the ohms isn't what determines that. It is a matter of the gauge ( diameter ) and length of the wire in question. A small gauge wire of a certain length can have the same ohms as a larger gauge wire that is longer. A smaller ( finer ) gauge ( larger numbers mean smaller diameter ) wire will burn out quicker and easier than a larger gauge wire.
The temps of the coils and eliquid are pretty important also. You don't want to exceed what either can handle. How much surface area the wire has ( based on the gauge and length of the wire ) and how much eliquid can be wicked to the coils will determine how hot everything gets. So it's the surface area of the coil in combination with how fast the wicks can deliver the eliquid that will determine when something ( coil or the juice ) burns...
Another area that wasn't mentioned... was what temp the eliquid itself may be able to handle without it tasting burnt or the flavor dropping off. As a rule of thumb ( that isn't always right actually ), more delicate flavors usually can not stand the higher voltages/wattage's as well as bolder flavors may be able to handle.
So... what wattage/voltage you end up using... will depend on how fast you are able to wick the juice to the coils. The more juice you get, the cooler the coils can stay and the higher the settings that can be used. Just because you can go to higher settings, doesn't mean that will give you the best flavor or the most vapor... many other factors come into play also.[/QUOT
:O (lol)