I was about to post the same link as Aadill. What I got from it is that Backstab is just another melee attack with a modified weapon damage value.
Doesn't this mean it should have the same damage distribution as regular attacks? I am at work and don't have a log available to me to compare regular melee with backstab damage distribution.
With a cap on the minimum damage and low damage piercers it now makes kind of sense to have a higher percentage of minimum damage backstabs with a 10 vs a 15 dmg piercer (not sure how to explain: a bigger part of the graph gets set to minimum damage the closer it is to that number). Before I always thought of backstab as an attack with seperate rules from regular melee. So now I am just wondering if they are infact treated the same in regards to atk & ac here on p99.
Nothing on minimum damage 52-59 though.
edit:
another link that might be interesting:
How is backstab calculated?
specifically:
http://www.brogett.plus.com/backstab_formula.pdf
While not the exact formula and way after our timeframe maybe someone can get some info from here.