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.