The FAR/AIM also states 20nm.
I am wondering why ATC did not reroute these planes in this area prior to takeoff and during their fights without the crew having to request a reroute or altitude change. It’s very clear that the storm 8501 entered was a large building level 4 or 5 thunderstorm wall running up to the east side of Singapore and then south a few hundred miles (see radar image for that morning). It was clear on the radar to the west of their route. If this wx in this region is typical for this time of year ATC must have a reroute action plan for aircraft in place. Lots of dominos caused this accident.