General rule of thumb as I've been told is add 2 to the ohm rating on the coil for max voltage. Any higher and you'll risk a burnt taste. Also this chart has helped me out. http://www.crimsonvapor.com/v/vspfiles/assets/images/voltagechart-lg.jpg
Edit: Find your ohm rating on the left and go right, try to keep voltage setting in the green(voltage at bottom of chart)
That chart varies greatly from one I looked at yesterday.
http://i.imgur.com/Ohn8o.png
Now I'm not sure what to believe.