@KasEV
It's rough but gets you close to what you have used up between %A and %B, in reality you could have used more energy as it doesn't account for any energy recovered during regen (if all the mileage is at speed on the motorway, this will be negligible, but it is worth noting). I would hope that the m/kwh calculation the car does takes this into account as well.
The "rate of discharge" as you mention of the battery should be linear, unless I've missed some reasearch on this, and will only vary based on energy amount demanded by the actions of the car user, be it propulsion or recharging the 12V battery as it provides energy demands of the other 12V systems in the car.