Voltage regulation comes with a set range of tolerances on input voltage and current draw. You can do a headroom calculation if you know what the regulator circuit voltage dropout is and subtract that from the design output voltage, then finally subtract that from the minimum input voltage to the regulator. If the power supply is designed correctly for the application and input voltage, you will have a positive number. That is the headroom of the secondary voltage of the power supply’s transformer. Finally use the transformer output to input voltage spec, and divide the result by that ratio to determine the input voltage headroom. That’s a fairly simplified overview, but is close enough for purposes of estimation.