ORIt's pretty simple once you know the math.
Divide volts by ohms, this gives you the amps.
Multiply volts by amps, this gives you watts.
Wattage is the power (heat or vape) that you feel.
For instance
3.7 volts divided by 2.4 ohms = 1.54 amps.
3.7 volts x 1.54 amps = 5.7 watts of power.
I've heard that 8 watts is the preferred vape but it's subjective, I prefer 3.7 - 3.8 for most juices in the Phoenix top coil.
A calculator:
Ohm's Law Calculator