Indeed, it is possible to very roughly estimate the volume of...

Indeed, it is possible to very roughly estimate the volume of stock that funds need to divest by comparing to previous companies that have dropped out of the index. Subtract off their regular volume from the volume traded during the month after the announcement, then compare their index weighting to NWH. Averaged over a large number of stocks you can come up with an approximate figure that's likely to be offloaded. When that many shares have moved over the normal NWH volume you should be ready for a sp bounce.
It's not exact, and you need to allow for increased trading volumes due to whatever reason caused various companies to drop from the index. Best reference points are companies with no significant news before or after the index removal that were simply overtaken by growing companies.