Charging overhead varies with a couple of things. 1) Charge rate. There is a certain amount of power required to do things like run the coolant pumps and things so at very low charge levels as a percentage of power coming from the wall, less is going into the battery. At very high rates of charge the system will have to run the coolant pumps harder and perhaps even engage the A/C which also decreases in percentage terms the amount of power going from the wall into the battery. Tom Saxton here did a study for the Roadster and determined that 32A was the sweet spot for maximum efficiency. Don't know if anyone's done the math for Model S. 2) Temperature. If it's very hot where you charge and/or you've just run the car hard so the battery is hot and begin charging immediately the car will use some energy to cool the pack. 3) Running the cabin HVAC while charging. Pretty self-explanatory.
As the car approaches 100% it will lower the charge rate so there will be more energy lost to overhead than at lower SOC but I doubt you could measure it w/ your power meter.