I’m pretty sure it doesn’t work like that.
First of all:
10% penetration is not necessarily the same as multiplicative 10% damage. I’ll explain later.
The blessing doesn’t give Cold Pen, like amulet affix or a passive from Sorc tree. It gives a chance to apply “Shred Cold Resistance” ailment. Which itself applies negative cold res (same like Snap Freeze) and STACKS to 20 (and with ice barrage you should be able to maintain 20 stacks quite easily on bosses)
AFAIK, resistances can be dropped into negatives, so, now, the actual damage increase depends on the initial cold resistance of the target:
- If base cold res of the target is 0%, then yes, you can treat Cold Pen (or any pen) as yet another “More Damage” multiplicator. With 40% cold pen you do x1.4 more damage, with 140% pen you do x2.4 more damage (which is 71% more damage than without Cold Res Shred blessing).
- But, assuming a target has 75% cold res, without penetration you only do x0.25 damage to it. With cold pen from Cold Snap and Passive you have 40% pen and now you do x0.65 damage to it. So, 40% pen actually results in 0.65/025 = x2.6 = 160% “more” damage! Which is fucking HUGE.
- Now, with 20 stacks from Cold Res Shred you have additional -100 resist on the target. Now you have 140% pen! Compared to 40% you now do 1.65/0.65 = x2.53 = 153% “more” damage. Which, again, is fucking HUGE.
Honestly, I’m not sure how much base cold res monsters/bosses have. If it’s 0% for everyone, well… this is boring, I must say. I would easily believe that target dummy has 0 armor/res/whatever. But bosses — seems unlikely.
But what I’m absolutely certain of, is that x20 stacked Cold Res Shred is DEFINTIELY more than 29% damage boost. It’s noticeable even on target dummy, that it’s somewhere around x2 boost.