"My friends C-Zero, year 2011, 14.000 km, averaged 148Wh per 1% SOC, so his value for 5% is 740Wh." The value shouldn´t be so high. I presume the car had recalibrated is capacity for a value higher then the original one. Now, your friends car will stop not with weakest cells at 3,6 V, but at 2,75V.