I thought I'd ask for other players best numbers when backstabbing. I'm only now using this ability in my current game and finding it can be a lot of fun with a stash of inviso potions. I just fought the King Sahuagin + cronies and when the battle started I quaffed a potion, he ran off and then I backstabbed him. I heard the sweet, sweet critical hit exclamation and then he just exploded. Quintuple damage, 150 points. This was without cheese, and no real battle-related buffs, using upgraded Daystar. I must say it was a pretty cool sight.
Potions of Power (red ones) are great for backstabbing ~~ lowers your THACO very nicely... But stabbing with a uber-sword, you don't really need that, eh? 150 is very nice, indeed!
I've seen 250 with a fighter/thief and the Staff of the Ram...I should imagine a berserker or kensai>thief would be able to go way higher. And then of course the potential with a hacked kensai>assassin is even greater, with the higher multipliers assassins get.