To be able to calculate the input wattage of your PSU, you need to know your Power Factor, if you're using two multimeter to perform voltage and current measurement then your result will be in VA not Watt.
I have a plug in meter that measurs watts, 13a plug one side 13a socket the other, I just do't understand why the input is rated at 408w (by calculation) but the output is only 65w (as stated)
Your specification is not given in watt but in Volt and Ampere. So doing the math of V x I is wrong if you want to know what is the maximum power. Refer to post #3.