Pardon my electrical ignorance: the variac puts less than 120 v. into the transformer, so the output becomes less than say the 20 v max.(if a ZW) ? So you have to experiment to find the voltage to adjust on the variac to get the desired output from the ZW?
It's all proportional. If you have a train transformer set to put 16 volts on the track with it plugged into the usual 120V outlet, when you turn the outlet voltage to 90V (using a variac) the track gets only 12 volts. The variac reduced the input to 75%, so the track gets 75% of what it would get at full power in.