Duty of care under FIS/BS - sorry, it's easy - less than 1nm and 500ft separation call it, more than that, don't. Think about what it's like in the cockpit, the guys has asked for a FIS/BS for a reason and when you're fg, a miss-distance of 1nm is HUUUUGE! I think people are over-analysing just because something has been written down talking about duty of care. It's always been there.