This is probably not new to many of you but after experimenting with backstab i think i have pretty much figured out how the formula works.
Here's a summary of the Backstab data points for Adventurer, in case you missed it from the patch notes:
Adventurer
Enabled: 340
Damage Multipliers: 540
Critical Increase: 625
These are unbuffed and unimplanted values.
Now over to how backstab damage is calculated.
The following numbers assume you have over 625 backstab skill.
The minimum amount of damage that backstab can do is the same as the minimum damage of your backstab weapon.
Lets assume that you do 1k damage minimum on Adonis hecklers and that you critt for 2k.
Your backstabs will be:
1k Regular damage
2k Critt damage
4k 2x Critt multiplier
8k 4x Critt multiplier
10k 5x Critt multiplier
Backstab is capped at 13k
One thing to note is that every +add damage item is very useful for making large backstabs. The +55 damage from the full ferocity of nature line for example will give you another 275 dmg to your max backstab.
/shelana