Ann: Investor Update Presentation & Confirmation of FY18 Guidance, page-336

It's pretty difficult to work out in advance what the index will look like. You have to do 6-month averages for not just the stock in question — Vocus — but also all the other ones just above to see who else might drop out and save Vocus, and all the just-below ones to see which ones might enter the index and push Vocus out... massive amounts of work!