Yep, that is how I do it too. Doing it this way takes into account all of the electric power train inefficiencies. The in car charger, battery in/out, motor controller, motor, ...For a PHEV owner driving exclusively on plug-in charge, something like this measures the electricity consumed by charging: https://www.amazon.com/P3-P4400-Electricity-Usage-Monitor/dp/B00009MDBU/ref=asc_df_B00009MDBU
Your odometer measures miles (if you trust it, otherwise, look for a highway that marks off measured miles).
And then the math for "mpkwhr" is just the miles divided by the kwh reported by the meter.