Ohm's and volts equaling wattage can be a bit confusing at first. This link might come in handy for you. Just fill out the Ohm's of your cartomizer and the voltage that your battery is producing. Then hit calculate. The wattage that your vaping at will show up on the right. Some say 6 to 8 watts is a their sweet spot, But I go higher than that sometimes. Zeus is constant 5 volts, the DAV is a variable volt, meaning you can adjust your volts to your liking.
Ohm's Law / Watt's Law Calculator