It is 553 on P99. If I look at my archived log file from 2019->2023, I have 489 instances of someone backstabbing for 553. I have 250111 instances of someone backstabbing someone, for about a 0.2% max backstab damage hit. It is a little bit better than this, I just searched for lines with "backstab" so will get like text from people with backstab in their name and NPC backstabs and stuff too.
IIRC the damage bucket code is kind of complicated, but I believe you basically are rolling a weighted 20 sized die to get one of 20 different damage interval numbers for any attack. You and your opponent roll an attack and AC roll, and subtract the AC roll from the Attack roll. The difference is divided by the average of AC+ATK, rounded to the nearest integer and added to the offset from 10 on the 20 sided die.
If your ATK is not sufficiently high relative to the opponent's AC, you can't reach the top damage intervals. I've seen this on AoW, where we had Yetarr swinging at hit with like 1000 attacks and only 17 different damage numbers, because his attack is too low to be 10x the average between the attack and AC even if the AC roll is 0 and the Atk roll is near maxed. Without high attack buffs, if attacking a high AC target, it can be impossible or exceedingly unlikely to reach the highest damage intervals.
Looking at my logs, by far the most common band of 553 backstabs I had was during a lengthy Halls of Testing situation where I brought two stacks of emeralds and asked a Shaman to Avatar me continually so I could see how high I could push my DPS. Its one of the few times I have had the full epic bard/ranger/avatar +ATK package.
[Sun Mar 19 21:54:30 2023] You backstab a tigerclaw racnar for 553 points of damage.
[Sun Mar 19 21:54:30 2023] You have slain a tigerclaw racnar!
[Sun Mar 19 21:54:45 2023] You say out of character, '/GU A tigerclaw racnar in 121s, 47161 @390 | Jayya 12875 | Cained 6180 | Mackn 6034 | Kaelton 5598 | Barbarisater 5067 | Darstan 4858 | Perridot 2982 | Evolaa 2497 | Zobekab 559 | Lebtik 511'
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