The majority of vapors, statistically, use a wattage setting between 5 - 7 watts.
Voltage-wise, a simple formula is to take the ohm of the heating coil and add the number "2". (ie 2.0 ohm + 2 = 4 volts) That will be an ideal "starting point". Adjust lower or higher to taste.
I tend to set my voltage to the specific flavor that I happen to be vaping. I start with a lower voltage and work my way up in increments until I begin to taste a little burnt flavor, then I back down about 0.4 volts. I've found fruit flavors are best in lower voltage and others in a mid-range of 3.8 - 4.4 volts. I very rarely go higher than 4.5 volts.
I use a Provari (regulated mod) and typically use a resistance 2.5 - 3.0 ohms.