I have learned to back down on volts/watts if I even get an "inkling" that the coil is so hot that it may want to singe the wick no matter what topper I am using. I dont care what it reads as far as a number, with the exception that what ever it reads at my "sweet spot", thats were it will stay with that certain coil/ohm (until the ohms go up). I say al that to say that I do go as low as posible and still get a good vape because I will get more flavor like this. The higher I go, the less flavor and the more bland or burnt taste I get.
Then again, if you havent noticed yet, your ohms tend to go UP the longer you use a coil, I am guessing from a build up of gunk on the coil itself. So then Its time to "bump it up one"...if needed.
I wouldnt worry about the numbers or formulas, I would "taste" what your vape is telling you. I dont like my juice getting "discolored" from too much power going to the coil, so I tend to "back off" for the flavor and for the longevity of my coil.