I've just put in my most recent tankful as a partial, because Shell have just introduced assisted fuelling, which is unheard of here. So I wasn't able to vent&brim, & hence I wouldn't get a reliable mpg figure (I guess it was at least 4 litres short). By putting it in as a partial it should correct itself next time.
Perhaps that's not the right terminology. It's when someone from the garage does the refuelling instead of the driver. Hence why it's not possible to get a consistent refuelling compared to previous ones where I always brim&vent.