I just wanted to make a quick note about the “buffer” capacity. A battery is not like a gas tank, which has absolute full and empty points. The battery manufacturer decides minimum and maximum nominal voltages for the battery, and the battery’s nominal capacity is what you should get if you discharge from the max voltage to the min voltage. There’s nothing preventing the battery from charging or discharging to above or below these voltages, it will just result in a larger capacity and a shorter lifespan for the battery than the manufacturer specifies. Likewise, the battery’s lifespan can be increased and capacity decreased by setting more conservative voltage limits. MINI has evidently chosen to set more conservative voltage limits than the battery manufacturer, resulting in a smaller usable capacity than the battery’s nominal capacity. You might be able to extend the lifespan further by keeping the battery to an even tighter voltage range, but at some point, you’ll reach a point of diminishing gains.
I don’t know much about the MINI’s particular battery chemistry so when I go on a trip I usually leave it at 80-90% in hopes that it’ll extend the life a bit longer. But I leave mine plugged in most of the time so that probably doesn’t make a difference.