Volts or watts is really up to you, if you adjust volts you are setting the voltage. If you adjust my watts your mod adjusts the voltage to give you the watts you want. Stepping the voltage by .1 volts is going to give you a finer adjustment than adjusting wattage by .5. It really is just a preference thing. You can set wattage, if you like your different juices, vaped on different atties, all at the same wattage. I have found that different flavors will taste better at different power levels. Coffees and other easy to burn juice need lower settings than some of the juices that can take more heat.
I know this probably does not help much. There is a chart you can follow for a range on volts or watts for different resistance atties.
http://www.vaporsworld.com/wp-content/uploads/2013/04/power-vaping-chart-lg.jpg
You mod should shut off when battery level drops below usable. The previous mentioned suggestion is best. Start off low then step up little by little till you get the best flavor, if you start to taste crispy juice then turn it back down a little.