You set either the volts OR watts, you go with one at a time, not both at the same time -- if that makes any sense. So, if you're in wattage mode, set that. Leave it at that.
As for the "highest" you should go in watts or volts, start low and vape. If that doesn't do it for you, turn it up a little. Vape. Keep repeating that until you get a slight burnt taste or a harsh vape. Turn it down just a bit and that will be the sweet spot for that tank-coil-juice combination. Each one will be a little different so you'll just adjust each time. I've noticed that fruit flavors like lower heat while bakery flavors/coffee flavors like it hotter.