I've never tried to quantify, but now that you ask I suspect I will feel the need to do so ...
One thing you can do (if you have the nav display) is to have a passenger press the Zero Emission (physical) button on the lower right of nav display, and then tap the green Energy Info (digital) button (last one on first row). Energy Usage is the display you want. As you drive, you can call out "five white" or "two green" and the passenger can note the kW used/generated on the "gauge." You should be able to get at least a rough calibration doing this.
Let us know!