Exactly what's the deal with the new Ambush/Bstab?

[russ]

I seem to be making a lot of topics recently...

Ok I know that it's getting buffed and that Lethality doesn't affect it anymore (at least Ambush) but how exactly does it work? Is it a damage increase over the board? Or do you need Attack Power+ items for it to really shine? How exactly is the damage calculated?
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My understanding is that, before the patch, on a rank 6 ambush, you would do:

[(weapon_damage x 3) + 290] * (1 - enemy_dmg_reduction)

With the current test patch, you would:
[((weapon_damage + (attack_power / 14)) x 3) + 290] * (1 - enemy_dmg_reduction)

The difference is mostly significant for agility rogues, giving a massive boost to their opener damage. Anyone with significantly buffed agility will have a major boost from this change.