An external meter will give you the amount of power supplied to the car. The battery charge will be less because some of the power will be used by car systems such as battery conditioning (heating and cooling).
You can estimate power added to the battery by checking the before and after battery percentage and multiplying the difference by the battery kW/percent. In your case, the 75 kW battery, 1% charge represents 0.75 kW. So, if you add 20% to the battery then the power is 20 times 0.75 which equals 15 kWh.
