The input voltage should be measured on the board itself. It can be measured either at the input terminals or across the input capacitors. In a high power design there is a significant amount of input current. If there is even a 1mOhm resistance across the input connectors for the design this will create a voltage drop between the supply and the actual input voltage into the design. As the output power is increased this voltage drop at the input also increases. Therefore to get an accurate measurement of input voltage for an efficiency test it is important to measure the voltage at board level.