Originally Posted by
Suspicious
First of all, the talent that converts spirit to hit rating is in the second tier of the shadow tree; and you should never EVER go that deep into the shadow tree as disc. There is a major glyph that will hit-cap you for atonement purposes.
Second, the spirit>mastery>haste>crit idea is FALSE. This is usually pushed by people who gear/reforge themselves based on crappy simulators like Mr. Robot. The fact of the matter is that as disc, your primary form of regeneration will come from abilities that restore mana based on how big your mana pool is. Therefore, the better geared you become, the less spirit you will need, usually. Therefore, there is a limit on how much spirit you need, and this is roughly based on what your ilvl is (since every piece of gear of the same ilvl has the same intellect for that slot, you can think of it that way). Without knowing how well you are geared, it is tough to tell how much spirit you will need, but 2200 will be the absolute MAXIMUM you need if you are in Firelands gear. A lot of people, as they get more heroic FL gear tend to move towards 1800 spirit. After that point, spirit becomes USELESS. Furthermore, the remaining secondary stats all benefit from one another in ways that are very complicated to model... but suffice it to say that haste, crit and mastery are all roughly equivalent.
Therefore, the answer to the question of how much crit you need will depend on how well geared you are, because the crit becomes less valuable if you have less mastery and less haste. It's the proportions on that synergy that are tricky... but you can see this easily because more crit gives you more critical heals... which gives you more aegis... and with more mastery, your aegis will be bigger. Same with haste... more haste = more casts = more crits = more aegis... and so forth.
So it's pretty hard to answer this question without knowing what kind of gear you have, but as a baseline, I run with 19% crit as a 384 ilvl disc priest, and that feels pretty good to me. The parses showed me that my aegis healing went up a lot when I reforged my spirit into crit (to hit 19%) instead of stacking up way more haste and mastery, and resulted in more healing overall.
That's my 2 cents on the issue.