There is only one INMARSAT covering ACARS coms over the Indian Ocean. It is IOR located at 64 degrees East, 35890km above the equator, as shown in the Malaysian diagram.
So angle (or equivalently distance) calculations must be being done by using just the one satellite.
I think their diagram shows that they have a very accurate method of calculating that distance. I understand that this can be done via ping latency but there may be other methods.
The distance from the satellite so calculated has given them the 40 degrees altitude as shown. I calculate that this equates to 4840km distance at sea-level around the earth from the point directly below the satellite.