Ann: Becoming a substantial holder, page-39

At the moment I think it is more like $650m, but remember they have a buffer zone to reduce turnover. If you are already in, as long as you stay above 221st, you can stay in... you can also stay in if you fall below 221st but there's no eligible additions (ie, outsiders that rank better than 179th). Of course, the docs provide for the index committee to apply discretion and do something anyway, so that's why you can't know for certain what the index changes will be. There's a whole industry of add/delete arb traders around these sorts of events.