The only way would be to measure the total fills vs the total mileage. The trip computers are all a bit bogus
I did exactly that with my final ICE car, over a period of 4 years. It was a Skoda Octavia vRS diesel, so a 2.0 TDi auto & of course not really comparable to a M3. Its overall average was 34.73 MPG. The computer always reported low- to mid-40s, which was why I recorded the data myself to see how far out it was.
For my M3LR my gross comsumption, according to TeslaMate, is 330 Wh/mi. So that would be the figure most comparable to my diesel log for the sake of this discussion. Net consumption reported by the car is a worthless figure to me, I'd rather look at total gross cost including charging losses.
With today's diesel at 165p (average in my area I think) - 21.56p per mile
With my home electricity tarriff averaging 26.53p - 8.75p per mile
If I manage my charging more actively (which I do), the average is probably closer to 20p at a guess, which would be 6.6p per mile
I try not to think about it too much, but my tariff was 5p overnight until last week, which was 1.65p per mile...!
(Agile average price is 26.53/KWh for March so far last time I checked)