FWIW This charm seems to bottom out at 12ac 125HEM. The highest I have seen it is 31AC 312HEM but will try to see if I can push it higher.
Edit: Highest appears to be 37AC 375HEM.
The scaling is kind of weird and doesnt exactly follow the description. The charm actually gets stronger based on the total health difference between you and the rest of your party. So if, for example you have 10000 HP and you group with 1 other player with 30 HP the charm will be at max stats. Unfortunately this means that when players are just below you in total HP the charm doesn't actually get that much stronger if at all. When soloing the charm will also always be at minimum stats.
Edited, Apr 12th 2018 1:08am by Bobbybick