Fair market value on 31 January 2018
Regulation & taxFor anything listed on 31 January 2018, the highest price quoted on a recognised stock exchange that day — or on the last preceding day it traded; the net asset value of that date is used only for a unit that was not then listed.
In plain terms
Published historical data that will never change again, and worth writing down once for every holding older than February 2018. It is a ceiling on the substituted cost, not an alternative purchase price you may use in either direction.