I must stress that taste is the most important thing. You can use charts and calculations to get a starting point, but then you adjust up or down to find your "sweet spot." If you go strictly by math, you may not be happy. Example:
I am current vaping a carto tank. The carto is 2.1Ω, the voltage is set at 4.2v = 8.4w. That's a bit high for me, but with this juice is yummy. I have another device with a 2.6Ω head, that when filled with the same juice, started frying over 4.0v ≈ 6.2w. Why such a big difference? Why is it Tuesday?
If I was new and just using math, I might post on ECF that the juice was frying over 6w, which isn't supposed to happen. I would get all sorts of responses: clean the head; take the wick apart; lower/raise the nic/vg/pg levels; get a Vamo; get a ProVari; spin around 3x at midnight while praying to Thor - whatever. The ultimate answer? Turn the voltage down.
You don't have that option with a fixed voltage battery device, but the concept is the same.