Why would you need any complicated math? It's simple enough:-
(VA Power Out / VA Power In) * 100 = % Efficiency
To measure these you could simply hack an extension cord so that you can get a multimeter and ammeter into the input. Measure the same on the output and you have the figures you need.
You could then load up the output to anything from 0-100% and see calculate a table and graph the results of efficiency over secondary load.
The problem is measuring VA Power In, since it's an AC quantity on the power mains. You have to break the 120/240VAC connection to measure current, unless having some equipment I don't have. Plus my DMM does not measure AC current.
Also, it's been so long since college, I forget. Can you simply multiply Vrms * Irms, ie what the DMM/meter would give you, and get the correct value for AC power, or do you have to take the relative phase-shifts into account? The latter, I think. ???