The RULE OF THUMB(so named because we ProVinati like to use our thumbs for the button, it's classier that way, don't ask why), is that you take your ohm rating, and just add 2 to it, then adjust up or down from there to taste.
The reason is, this gets you roughly close to 8 watts, which is the sweet spot for a lot, but not all vapors out there. Once you get a better feel you'll come up with your own random number to add to it that is closer to your ideal
vape.