And, do not forget, all of this (tables and computer) cover the worst case scenario: a sudden decompression at high altitude.
Are you sure of that? The majority of the FAD calcs I have seen are simply based on a pressure altitude of about 8 000 ft.
In my book, 24 hours is more than enough unless you have been doing multiple deep dives.
The problem is that the generic rules must cater for the worst case. There is a reference in one of my textbooks (JSSG-10 Crew Oxygen Systems Handbook) to a case occuring in flight 4 days after the dive. Trouble is that the textbook gives no other details, so one cannot be entirely sure of the situation.
Somehow I don't see anyone changing the generic rules in a hurry