Ok, I fixed the calculations for Spirit Shell in the spreadsheet. I created an input area on the "Special Calcs" sheet that lets you pick a breakdown of spells used during SS as well as the percentage of max SS uptime you achieve in the fight. I am optimistically assuming that you would only cast Heal, FH, GH, or PoH during SS, and that you would cast back-to-back for the whole duration.
I was able to calculate some thresholds of spell usage where Mastery becomes more valuable than Crit for Heal/FH/GH/PoH. What I mean is, lets say you cast the spell 100 times. For Mastery to be stronger than Crit, you will need to cast it at least X times while SS is active.
Heal: 100%+ (Crit > Mastery always)
Flash Heal: 60%
Greater Heal: 100%+ (Crit > Mastery always)
Prayer of Healing: 25%*
*This assumes you hit 5 targets every time. The number goes up if you hit less targets.
So, as I have mentioned before, The relative value of stats can change dramatically for Discipline based on how you use your spells.
I tried creating a theoretical breakdown of spells, just to see how it would turn out. A "problem" I am having with the model is that the Spirit Shell calculation is extremely optimistic. Lets say you used Flash Heal 10 times and Prayer of Healing 15 times in a 5 minute fight... right now my calculation would assume that you only used those spells with Spirit Shell up, since that would have been possible. Thinking to myself... I know that I would use those spells at other times during the fight, at least once in a while. So, I also implemented a max percentage of the spell usage to allow during Spirit Shell.
This should give the necessary flexibility to calculate when Mastery will become stronger than Crit, and vice versa.
So, some notes on how to decide if, based on your spell usage, Mastery or Crit is stronger:
1.) How often do you use Flash Heal, and, how often is that done without Spirit Shell active? You need to limit your usage of this spell outside of SS to make Mastery more effective than Crit.
2.) How often do you use Prayer of Healing? Mastery is good for this spell. As long as you are using it about 25% of the time with Spirit Shell, Mastery will give you better throughput than Crit.
3.) Mastery is awesome for PW:Shield. The more you use PW:Shield, the better a stat it is.
4.) Mastery is weak for Heal, GH, Renew, PoM, Penance, level 90 Talents. If you aren't using PW:Shield, Flash Heal (with SS), and PoH for 60% or more of your spells (not total healing, this is spell usage), Mastery is going to even out or fall behind Crit and Haste.
5.) Divine Aegis scales better with Crit than with Mastery.
Lots of interesting stuff I'm finding as I refine these calculations. I will be interested to see what is actually possible in different situations, based on the limited Mana. Mastery is very effective for PW:Shield, FH, and PoH - which are all costly spells. Will we have the mana to keep up a spell usage pattern that allows those to make up well more than half of our spell usage? I can't answer that yet, since I don't have a level 90 disc. priest on beta.