IT would depend on how you want it to work especially how often you want it to work.

Back stab in general is designed to be hard to pull off (IMO it should be made easier).

Sneak attack in 3e is easier to pull off but there are lots of dumb ways to negate it. However can be done every hit if you fulfill the prerequisites.

Sneak attack in 4e is even easier to pull off and has no ways to negate it. Can only be done 1/turn (after errata).

sneak attack in 5e is even easier to pull than 4e and is hard to negate and is still 1/turn.

Pre 3e it seems to be assumed that generally you are not back stabbing, 3e seems to assume that you can get more sneak attacks and is willing to give you a lot of once but has lots of ways of preventing it from working, 4e and 5e essentially are designed with the idea that you will get to sneak attack almost every turn but only once per turn.


Personally I have not done it yet but I would suggest the 5e method as a good starting point. The damage will feel good and it it is not annoying to use and the thief class is a bit lacking in AD&D in combat unless you go out of your way to make them work. Perhaps it would work to use the 4e damage values (2d6-5D6) and have it once per turn but use 5e style of making it work (only needing an ally adjacent to the enemy to pull off).

Makes for a starting point though I am coming in from the idea you want these to happen almost every turn unlike the original back stab.