sorry what about my battery says 3.3v/4.8v whats best
The reason it allows you to adjust from 3.3 to 4.8 is so you can SLOWLY adjust it and experience the difference yourself.
Nominal voltage is 3.7 volts so you can just start there and adjust up or down; if taste is muted you adjust up; if taste seems a little burnt or you hear loud popping sounds you adjust down.
Also, as you use a tank, and coil, sometimes you start off at one setting, but as you use the coil more, you find yourself raising the voltage/power.
Dual coils also tend to need more power than single coils; roughly speaking they need twice as much power because there are two coils; it's like vaping 2 times at once; on the minus side, you end up using up more $liquid$ faster.