hmm i was only hiting like 500 at rank 1

Backstab R3 gives you 160% damage if you land your hit to the back of victim as far as i am concerned it doesnt work on rear sides

Damage indicated caps at how much health the person you've done the damage to has.
As inquisitor I've backstabbed for 41 damage after level 3 backstab. It only shows how much you've done, not how much you are able to do.
Hope that is helpful

I usually backstab approx 2,5k if its landed correctly from behind and with PA the change might not be very noticeable since BS powerattack might kill someone by one hit and then it shows you their max HP...for example like 1,800 against low witch. But I think Ive been hiting some orcs from behind for more than 3k, but I am not sure...But definitely I attack much higher from backstab with normal attacks, since normal attack in front is approx. 160, but backstab is more than 400.

But what makes me really annoyed is the radius from which such a backstab is working...its like 60 degrees only, whereas it should be 180 as in any other game, where you use "backstabing" spells. Now everytime when you move just a little bit on the side, it is considered as a normal attack and therefore very low. This makes this main inqs ability very hard to use, since its very easy to just turn a bit or move slightly and backstab bonus does not work, also the enemy is not just standing idli without any movement. I am not sure, if it just me, but I think this should be adressed in any patch soon.