For example, my company uses 431nm for their EDTO threshold distance (60 minutes) & 823nm for their 120 minute distance.
My understanding is that the aircraft drifts down, so the initial TAS at altitude is high, it slowly decreases as the aircraft descends, the likelihood is that the aircraft won't have reached the level off altitude in 60 minutes, so you will still have a slightly higher TAS, but once the aircraft levels off, the TAS for the single engine speed will be at its lowest, so basically 120 minutes is not automatically double the 60 minute distance nor is 180 minutes 3 times the distance.
This is different to the methodology used by Jeppesen flight planning where the aircraft is immediately considered to be at 10,000 feet immediately following the event.