Some things to remember!!!!
1. The charts were written as guides. They were based on the type of clearos / cartos available at the time. As many of you have found out, watts don't mean the same for every topper.
8 watts on a Kayfun barely gets it going. 8 watts on my Mini Protanks almost guaranteed burnt flavor. Why?? Because the Mini Protank uses microscopic wicks and have very high airflow which would limit the wicking ability of the wick that was there. Therefore the 8 watt coil would quickly dry out the coil.
On a kayfun, you have usually a tighter draw and massive wicks (compared to the Protank). Now more fluid can get to the head and the atty can keep up.
2. The charts were based on single head attys
Dual head attys hook their heads in parallel. In a parallel circuit the voltage drop across each leg is constant and the resistance in a parallell circuit is not additive. I'm not going into the math. But it works out that a dual head coil is actually made up of two heads in parallel that are twice the resistance of the overall resistance seen or printed on the head.
So a 1.9 dual ohm coil is actually made up of two 3.8 ohm coils connected in parallel. If I used a VW mod and set it to 6.0 watts, then it would calculate the voltage for 6 watts based on 1.9 ohms total resistance, but only 3 watts would go to each head. This is why you can run dual heads higher than single head equivalents.
3. Variable Voltage (VV) and Variable Wattage (VW) are two mutually exclusive ways to get to the same thing.
A variable voltage device simply delivers the voltage you dial in to the coil. It doesn't care what the coil resistance is. (some do, but for the purpose of this explanation it's not important). If you change to a different head, then you will need to change voltage to get the same wattage to the head that you did before. Variable Wattage devices will measure the head resistance and AUTOMATICALLY adjust the voltage to produce the selected wattage.
This seems cool. And if all you vape is the SAME type of atomizer (i.e. protanks, iClears, etc). then Variable Wattage can limit how much fidgeting you do on a device. If I put in a Protank with a 2.5 ohm coil and then swap it out with a Protank with a 1.8 ohm coil, a Variable Wattage device will allow me to set a preferred wattage and each protank should give me the same vape. With Variable Voltage, I'd need to adjust voltage between switching the two tanks.
However, if you vape DIFFERENT style tanks, then Variable Wattage gives you no advantage. As we discovered, 8 watts on a ProTank is miles away from 8 watts on a carto or 8 watts on a Kayfun. Thus the "set it an forget it" advantage of a VW device is removed.
Hope this clears some things up.