Since it was brought up in the first thread and is relevant to this discussion:
http://web.archive.org/web/200103051...aps/eqexp.html
It actually seems that the information we were working off of on the wiki (and Blue, I guess) was incorrect.
For those that don't want to click the link, it is an XP calculator (much like the one in the OP) that has the Hell Level mods dating from mid-Velious (long before the "smoothing patch") and they fit to the OPs data pretty well:
1-29: 1
30-34: 1.1
35-39: 1.2
40-44: 1.3
45-50: 1.4
51: 1.5
52: 1.6
53: 1.7
54: 1.9
55: 2.1
56: 2.3
57: 2.5
58: 2.7
59: 3.0
60: 3.1
This shows that every level from 51-53 is a hell level, every level from 54-58 is a double hell level, and level 59 is a triple hell level. These are different from those found on the wiki, so if I can confirm that these are what are used and are correct, I will fix the wiki entry to reflect this.