I finally got around doing some testing on Shield Block's critical block formula.
It's still
CTC% - 75% and it's not capped, which means that Shield Block will give more than 25% critical block above 100% CTC.
Testing details
Tests were done with 106.81% CTC against lvl 85 mobs in Told Barad. I gathered several mobs, refreshed Battle Shout, activated
[Vial of Stolen Memories], started combat logging and activated Shield Block. I used WoWCardioRaid to filter the combat log, discarding any blocks within 200 ms of activating Shield Block.
Over four such tests, I got 86 crit and 12 regular blocks, which is a 86.04% crit block rate. The expected crit block rate with an uncapped formula was 85.17%. If the bonus was capped at 25%, it would have been 78.36% (Hold the Line uptime for all tests was 100% due to fighting 5+ mobs every time).
What this means
Currently it means very little. It will start mattering more in the Deathwing raid. Mastery's value still drops after 100% CTC, but not nearly as harshly if Shield block's critical block bonus were capped at 25%. According to my spreadsheet, point for point of rating, it drops to about the same damage reduction over time as dodge/parry.
For now it's not possible to say which gearing approach is better. Here are pros of each.
1) Stacking mastery past 100% CTC
- Stronger Shield Block
- Smoother damage intake (which is to some extent more important than simply dps taken)
- A Deathwing raid where certain bosses have increased hit chances but others don't (like Baleroc has) would favor this approach.
2) Reshuffling mastery into dodge/parry after 100% CTC
- Lower dps taken outside of Shield Block (where it matters more)
- Certain boss fights favor avoidance