I've tried that too and I feel it's not enough. For my 2 assassins it's a range of 10 to 25 hp. Not shabby, but not the 'oh sh!&' effect I'm looking for.
In the ACKs 2e doc, they include the damage bonus in the multiplier.
There may not be a middle ground. Maybe I'll try to say that you multiply the damage bonus by the damage die roll to a maximum of your multiplier. That would give me a range of 15 to 75 hp damage for a lvl 13 assassin assuming a + 10 damage bonus and x5 backstab. (max is actually a 5 on the d6 which allows x5 mult. On the bonus)
This would actually result in assassins/ thieves using d4 or d6 weapons as they have a higher chance of increasing the damage bonus mult. I'll try that out on the 13 th when we play next.
I have two problems with that, personally.
First, the upper bound on the damage is enormous, far more than I would consider right. 75 damage is enough to one-shot a 16 HD creature with average HP, and the character had a 33% chance of doing that much damage with a d6 weapon.
Secondly, there's no reason for them to use a smaller damage die there. Rolling a bigger number is still better, and so you would still want the die with the highest average roll, which is to say, the biggest die you can get your hands on. Unless rolling above your multipliper penalizes you in some way, which seems....like a bad incentive to me.
Have you considered including class damage bonus, but not any other damage bonuses, in the multiplier? This would allow assassins to have deadlier backstabs than thieves without bringing the maximum damage up to 'one-shotting dragons' levels. (Although that said, assassins already have deadlier backstabs than thieves, thanks to their ability to use any weapon.)