Turning 150 into 3k is a 20x modifier.
When I take my assassin and give her a 100 base damage dagger. I get 41% more damage on it from my attack stat making it 141.
1000% weapon damage using twin fangs, 2020% after critical damage bonus which sits at 102%. Add your damage bonus passives for now I'll stick with only cull the herd and it's 3030% weapon damage. 3969% weapon damage after my 31% flank damage modifier.
39.69 * 141 = 5596 damage. On my 100 base damage that's almost an 56x modifier. That's a pretty painful critical flank attack. Now add the fact that I have stealth up almost all the time thanks to "I was never here" and stealth adds another 50% damage. Also guarantees that 102% critical damage I'm running with to kick in. Of course you have a lot more damage passives and sometimes no cull the herd instead. Enemies also have a small amount of armor that reduces your damage after "Base * Attack" in the calculation. Some of that armor you ignore from stealth due to a passive. On the other hand my dagger is bigger than 100 base damage.
That 20x modifier is pretty easy to get and doing 5k with twin fangs is a piece of cake especially talking about a 150 base damage dagger.
Base crit damage is indeed 40%. Base flank damage modifier is 25%. Skills use your damage and not DPS this is obvious and why lower DPS weapons are sometimes preferrable on warriors.
Edit: Seems I accidentally looked at the wrong weapon as my 2nd weapon. I actually have 41.5% attack, 109% Crit damage bonus and 37% flank. Using 2 white daggers.