Yeah that link is pretty well known. Its not precise tho. It's good for areas to level in. For example, if you wanted to know what the mlvl for hell Urdars in the river of flame, that list says "77" for that area. Maybe I'm missing something here, but hell Urdars are lvl 82. In norm they are 50 lvls below that = 32. What you can do is go to the battle.net/diablo2exp site and click the left side tab for "Monsters". Then look at the normal mlvl. As they say in the link, +25 for Nm mlvls and +50 for hell. Do NOT bother with the LoD nm and hell mlvl numbers, they may reflect the actual difficulty and mlvl we are experiencing here in classic, but the experience calculations will use the old numbers, which seemed to have never changed since June 2000.
I've asked this before: what happened to the Darkness (monsters) site? There is still a link on the left side of the homepage here, which now just says "Monsters" but it takes you to a fairly uninformative partition. But wait! there is a link to the "darkness", which alas, takes you to a WoW monster site, arghhh!