After you've subtracted the effect of thousands of hard to measure variables like the slope, the tyre rolling resistance, the resistance in the wiring, the magnetic flux saturation in the motor, the accuracy of the current sensors, etc, etc, etc...
Not thousands but a few and most of them are measured anyway, like slope with a gyro or battery drain.
No need to over complicate it just measure the amount of regen the more you get the more the car weighs.
