Thanks StormyNight 3965 for the post of satellite angle diagram.
The angle of altitude of satellite INMARSAT IOR (which is as I mentioned the only ACARS one covering this area) at 64 degrees East, as viewed from a place on Earth, which is what the lines show, can be calculated from the distance from observer (or pinger) to satellite.
This, in turn, can be calculated very precisely from the time taken to negotiate a single ping (which has some back and forwards messages).
So they have calculated such an angle of 40 degrees for the last ping.