A Variac provides an output voltage that is a ratio of the input voltage. The 3PN1010B-DVM and the 3PN1010B are the most common models used with guitar amps. They provide an output of 0-117% of the power source voltage. This equates to a nominal 0-140V output with a 120V source.
Many guitarist that perform on stage use the 3PN1010B-DVM since it will display the output voltage at all times. Power sources at many venues are not very stable. The built in voltmeter allows the guitarist to readjust the voltage at any time for consistent results. This is much safer and more consistent since the voltmeter is built into the Variac.
More casual users often use the 3PN1010B. It has a dial that is marked 0-100% of rated output (Approximately 0-140V with a source voltage of 120V). You can estimate the output voltage from the dial or adjust by the sound. To get the precise output voltage that you desire you would need to measure the output voltage with a voltmeter. Dangerous voltages are involved with these measurements and they should be made with great care. If the source voltage changes you will need to readjust the output voltage in order to get the same result.