Purpose
This calculator is for figuring out the optimal resistance range for your regulated mod (variable voltage or variable wattage advanced personal vaporizer) based on its maximum output current, voltage and power.
Select your mod from the presets, or type in its limitations.

With most VV/VW mods, a low resistance is not required for high power output, so the minimum resistance may be less important than you think.

At the median resistance, the mod should be operating well within its voltage limit, as well as its current limit.
If you are unsure about the quality of your circuitry, this resistance may be the safest bet for achieving maximum power.

The maximum resistance usually gives you the widest adjustment range.

Between the median and the maximum resistance, there is a sweet spot of sorts, where many mods will give you a good balance of power and adjustability without overburdening your circuit board.
Note:
Some mod manufacturers exaggerate the specs.
Others don't publish the limitations of their products, so data from reviews or forums have to be used instead.
Mods also have individual variance.
Don't take these numbers as gospel; YMMV.
If you have better data, leave me a message.
Results
The minimum resistance you should use is typically determined by the PV's current (amp) limit.
The maximum resistance is typically determined by the voltage limit.
The median resistance is calculated by ignoring the power limit, and simply dividing the voltage limit by the current limit.
The "sweet spot" is between the median and the maximum resistance.
Keeping the resistance of your atomizer close to the "sweet spot" of your APV should give you a decent adjustment range (with most APVs), without necessarily pushing the mod to its limits.
Battery efficiency not calculated
Different mods and regulating circuits have different efficiency levels at various voltages. The efficiency is typically between 80% and 95%, but it can be lower.
With the power output being constant, it is the efficiency of the circuit that determines how long your battery will last.
There is no obvious way to know if your device is more battery efficient at lower or higher resistances, unless the manufacturer actually provides the data (most of them don't).
The efficiency characteristics of a particular regulator is usually figured out by hooking it up to a test rig, or through good old fashioned user experience.