RE: FTSE 25020 Oct 2024 14:20
“which let Greencore move up the final position to be listed” – there was no ‘final position’ at that time, it was a direct replacement on 16th May. The company exiting was somewhere around position 160 in the FTSE 250 at that time, Greencore went in on that day at somewhere around position 210-220 (310-320 in the All-share). They then moved up to position 181 (281) by 4th June which was when the June quarterly rebalance of the FTSE was decided, so were never in the relegation zone.
FTSE 100 + FTSE 250 = FTSE 350 – that’s where the 325/375 decision levels come into play, at the bottom end of the 350. Position 250 in the FTSE All-share is largely irrelevant, other than for Algos in the FTSE trackers making small day to day adjustments.
This link below is the official FTSE 250 list from the London Stock Exchange. Financial institutions receive this data every day and make decisions based on it, and their other data feeds, every day. They don't need to wait for any unofficial updates. Companies move up and down in the official list every day but the overall list of constituents only changes once a quarter unless something else changes, as happened in May. The “monthly update” you refer to is only done by StockChallenge to decide which of their subscribers has won their fun competition each month, no other reason.
https://www.londonstockexchange.com/indices/ftse-250/constituents/table