Multiplying damage

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.