The charts (the ones I've seen) all basically show 8 watts being the "optimum" value for any resistance. Another, quicker way to get there, without having to memorize or reference any chart, is simply the "ohms plus two" rule. Take the resistance of your atomizer, add two to it, and use that number as your baseline voltage. This will give you roughly 8 watts, in the range of resistances that we use. Then adjust the voltage up or down, to taste.