I think you are supposed to GO fill up your car when it becomes half full, and fill it back to full, and not let it get down to empty. Not stop filling it at half full.
So you only fill it with 1/2 tank every time, but that is from 1/2 full to completely full, not from empty just up to 1/2 full.
At least, that's what I understand. If I'm wrong, perhaps someone will correct me.