When you set it to variable power the voltage is all handled for you. So if you switch it to wattage and find a power setting you like, just leave the voltage alone. The vamo will adjust the voltage for you depending on what resistance atty/carto you put on it. Depending on the time of day I go between 4 and 6.5 watts.
So go through the power range till you are happy and it should be consistent regardless of atty/carto.