like KulrMeStoopid says just start low and find your sweet spot I use mine at around 4.5 to 5.0 volts with a 2.2ohm head on kanger protank 2 @leetimmins
Start at 5 watts keep going up a notch until it's too much for you. then go back down one.
Voila you've just found our own personal sweet spot (for that juice) different juices and different people prefer different settings. Most people settle on somewhere around 7watts.