"Dry hits" happen when your clearomizer/RDA/RDA can't feed the juice fast enough to keep up with the power you have your battery/mod set at.
The rule of thumb for "sweet spot" is the same no matter what vape device you will ever own. Start low power and take a draw, bump it up a little and repeat. If it gets to warm, tastes funny or starts to taste burnt (dry hit), back it down.