Frost Mage Crit Soft-cap (in raid)?

First of all, we'll work out the percentage that we need from gear, and then convert that to rating.

The hard cap for crit; that is, the amount required to land a guaranteed crit without Shatter, is 100%. The Critical Mass debuff reduces this to 95% (so in answer to your question, no, it doesn't stack with Shatter). The "soft" cap; that is, the cap with Shatter, is a third of the hard cap, or 95/3 = 31.7%

You get 3% crit from talents, so we're down to 28.7%.

Now, buffs. You won't be usiAng Molten Armour, but there are other crit buffs. These can be obtained from a Feral Druid, Hunter Pet, Subtlety Rogue, Elemental Shaman or Fury Warrior. Hopefully you'll have one of those. This is a 5% buff, meaning that we're now down to 23.7%.

Finally, there is Focus Magic. This gives you a 3% crit buff if you're lucky enough to get it, which brings us down to 20.7%

The crit rating conversion at 85 is 179.28 rating for 1%. Therefore to hit the 20.7% "soft" cap you will need 3705.12 (rounded to 3706) rating. How this is spit up between gear, gems, enchants and buff food is up to you.

I may have missed something - if I have, let me know. Otherwise, hope that helped.

~Roxton.

P.S With regard to the 5% boss crit reduction, I have never heard of that myself. If someone can prove it to be true then please let me know.