I'd be interested to learn more about what the NDL times were.
My edition of "The New Science of Skin and SCUBA Diving" is from 1966
The NDL's
are
Ft Minutes
40 200
50 100
60 60
70 50
80 40
90 30
100 25
110 20
120 15
130 10
140 10
150 10
160 5
170 5
180 5
190 5
Looking above at these numbers and then thinking about the time it would take to get a hard hat diver down to depth at a maximum rate of 60' per minute you might see where the 130' max sport diving depth came from.
It is not based on science, it came from the speed of the hydraulic winches the Navy used. At 130 feet you can get a diver down in just a bit over 2 minutes, giving a working time of 3 minutes before Decompression sets in, which was considered to be the minimum working time a diver needed for any observations. At deeper depths, the working time was less then 3 minutes and the Navy figured it was not worth it for a NDL dive. The SCUBA agencies took the same view and set sport diving depths at 130'.
The Navy tables are very safe if you use a few rules, obay them exactly, and don't push them.
If you hit 101 feet for a second, you have to use the 110' table.
If you go one minute past the NDL, you do the deco required for the first deco stop. It is here that divers had problems, the depths of 70 to 90 feet are the bend me depths due to the high DECO times required for going one minute past the NDL
Depth 10' Deco required for 1 minute past NDL
50 3
60 2
70 8
80 10
90 7
100 3
110 3
120 2
As many sport dives are in the 60 to 100 range, they are the most lible to break the strict requirements of the Navy tables by going into required DECO.