Man.
I feel like I did when I first visited Anandtech years ago (had an account but forgot the pw, so made this one).
Feel like I need an electrical degree to understand this shit. Think I might just buy a Blu to learn, like I did with my used 286SX. That was money well spent looking back at it.
I can explain it because I was totally lost but now understand how it works.
Each coil has an "Ohm" rating. They call that resistance. Think of each coil like a water hose. The smaller in diameter the hose, the greater the resistance to flow. The higher the ohm rating, the more electricity that can flow through. This means you need more electricity (voltage and current) to achieve the same heat as a smaller diameter coil.
If you have dual coils, it sounds confusing because the resistance is divided in half. Lets say you have a 1.5ohm dual coil juice tank. There are really two coils in there, 3ohms each. The reason it gets divided is because the same source of current from your battery gets split between both coils. So half the voltage goes through one coil, the other half through the other. This means that each coil is only providing half of its rated resistance because only half of the battery's electricity is flowing through it.
So if you have a dual coil rated at 1.5ohms, double that rating to 3ohms and then use the vaping chart for 3ohms. Because you need to provide twice as much power from your battery to heat each 3ohm coil sufficiently.
The dual coil tanks come with voltage instructions. They are usually a little less than exactly double, likely because they wicks can't provide enough juice fast enough for two fully powered coils.
Rebuildable tanks can fully power two coils because people tend to use cotton as a wick which provides better wicking performance than the silica string used in stock tanks and coils.
A rebuildable tank can use more voltage even for a single coil than the vaping charts indicate, because the cotton is able to provide more juice to the coil than your average tank can. This is why you see people (like me) with Kayfuns with single 1.5ohm coils running them at 4.5 volts (sometimes higher). That's way in the red on the charts, but those charts were made with standard, factory tanks in mind. Not rebuildable tanks where you can have a better wick with lots of juice going to the coil.
Also, if you make a coil with a large diameter, a lot of current can flow through. In order to achieve a 1.5ohm resistance, you will need to use a lot of wraps of coil, because electricity will start to slow down and get resisted more if you have more wraps of a coil. If you have a large water hose, a lot of water can flow through, but if you twist it and turn it, then the resistance to water flow increases due to the length of the hose. Same with coils.
Using a fat wire with a lot of wraps will get you a low resistance, like 1.5ohms, but you have a lot of wire to heat up, unlike just a few wraps of a smaller diameter coil at the same resistance. Therefore, the resulting heat delivered to the coil is spread out over a larger coil surface area, thus demanding more voltage to achieve a similar heat as a smaller diameter, 1.5ohm coil with less wraps.
Hope this helps.
Also, when people mention wattage, that word wattage is a measurement of the total power output to the coil. So with the example above, the larger coil will need more watts than the smaller coil in order to achieve a similar temperature between the two. 13 watts might be a lot for a regular 1.5ohm coil made with skinny wire and just a few wraps, but a fat wire with more wraps will absorb that 13 watts easier and result in less temperature. Watts don't create vapor, heat does. So 13 watts spread out over a larger coil may not be enough. This is also why more voltage and wattage is delivered to fat coils, despite being a low resistance.