if you hook up a kill-a-watt meter to your device and if it indicates that the draw is 400w and if you have it hooked up to a 500w rated psu, does that not equate to a 80% load ?
No. 80% load is 400W drawn from the DC side. At 85% efficiency, the PSU will draw 470W from the wall. Assuming linear efficiency, which is not accurate, therefore these numbers are just estimates any way. But if the PSU has a nice graph, you can get a good idea of the efficiency for the particular load in question.