The ProVari doesn't have a function to set the wattage. When you change the voltage, the wattage changes, but you can't set the wattage separately.
What you set your volts at is technically based on the resistance of the device you screw onto it, but it really comes down to taste. Charts and math give you a rough starting point, then you increase or decrease the voltage until it tastes good to you.
On the fly, I take the resistance of the topper, for example 2Ω and add "2", which equals 4. If I set the the device at 4v it will produce 8 watts. That calculation is true for any resistance. Resistance + 2 = X, the device set to X volts will produce ≈ 8 watts.
8 watts is a little warmer than I like, so I dial it down a bit, then up or down to taste.