
It doesn't seem like the info provided by Terrance is accurate for FF2us. I have here monsterdata.txt from the Tower of Bab-il docs. It provides the level in two hexadecimal values (one byte). I wasn't sure if it was correct at first, since some of them seem a little high...but I double-checked it with the Final Fantasy Wiki and they all correlate.
Where it gets wonky is enemies with the boss bit set. According to the doc, the first bit is the boss bit, the other seven are for the level. So for example, Sandy (who we know has the boss bit set) has a value of 90 for her level. A straight conversion to decimal gets you 144...seems unlikely. The FFWiki on the other hand lists 16 for her level. On the other hand, if you look at a binary hexadecimal chart and take out the first bit, you end up with 16 after all. Is there any other option than to do it manually like this?
Anyways, to get to the main point, I did some thiefing with a < Level 30 Edge in the Underworld. Up against a Level 36 RocLarva, Edge successfully stole a Potion 5/10 times. That's 10x better than the 5% Terrance predicted. Similarly, against a Level 41 Gloomwing, Edge got an Ether1 on the first shot.
It doesn't seem possible that it's working according to the formula Terrance provided. Unfortunately, this is about where my ability to investigate ends...