I think 8-9k can be fine depending on your setup. It's going to depend on how much spot healing you have to do, which depends on how/what the other healers play. In general, your AoE healing will be better the lower your spirit is as much of it comes from Efflorescence and Wild Growth which scale with mastery but not with additional mana.
If your item level is above 520ish and you have the legendary meta, you're not going to need all the spirit you get on gear. In the current tier it's something to reforge away from, not into.
Diplomacy is just war by other means.
we are 12/14 heroic 10, without a priest, shamman or regen trinket. I tend to run with 12.5k for most fights, except thok, where i go a bit higher. Will probably run with more for paragons too, from what i've seen at least it's pretty mana intensive.
Had 13K, which worked fine until I one day carried flex 1-3 and started to oom, to the point of shutting down at Thok after second stack phase. Upped it to 14K now. Would like to have the blackfuse trinket to swap in on days like that.
This is at 558, no heroic experience, 11/14 sooN.
Is there a way to estimate the optimal amount of spirit to maximize healing output ?
Should a healer stack spirit until the marginal return of it equals the marginal return of another secondary stat ?
Sometimes the problem isn't that your spirit is too low, but simply that the task you're undertaking isn't doable at your gear level.
It's not that hard to compare one spirit value that you know works to another and seeing how much healing you're giving up and how much healing you're gaining (convert the spirit into mana regenerated, convert that into Rejuvenation as a best-case-scenario, calculate whether those Rejuvs heal for more or less than the healing you gave up to get that spirit), but there's no good way to simulate a "perfect" amount of spirit as it depends a lot on procs, the nature of the incoming damage and how those line up. A fight might require you to cast ten Regrowths to stabilize people and it might be long enough that you can expect ten Clearcasting procs during it. Will those Clearcasting procs happen when you need them? Some weeks yes, some weeks no. So how many of them do you expect to spend mana on?
Skill is also a big factor. Both your own and that of the healers you're playing with. Good use of cooldowns and procs can let you do more healing with the same spells, allowing you to spend less mana. If your entire raid plays flawlessly and you're lucky with proc timing, you could probably heal any heroic mode in the game with 6k spirit. In practice, you won't.
Diplomacy is just war by other means.
A very rough napkin math estimate:
Suppose 1 Rejuv heals for about 350k, then if you're doing more than 130k HPS, 1 Mastery is better than 1 Spirit. (700k/0.015/(3*60s))
Edit: or the other way round, if you do about 300k HPS, you would need one Rejuv to heal for more than 400k to make spirit equally strong.
On high-throughput fights (Thok hc), I'm usually somewhere at 450k raw HPS, so one Rejuv would need to heal for 600k to make spirit as good as mastery (in reality, one of my rejuvs heals for about 350-400k).
I always recommend reading Hamlet's blog post on spirit vs. throughput stats (although since this is almost 2 years old, the numbers are from the start of the expansion):
Why More Spirit is not the Answer to your Healing Problems
13.5 swapping in and out #14 trinket based upon fight... some fights hardly use any mana, other fights, I'm nearly Oom...also depends which Heroic team shows up...The one we down shit super fast or the wannabes whom we wipe on Sha of Pride 5 times. the later really makes Korean baby Jesus cry.
I am at 12/14 hc 25 and try to stay around 13k spirit. That works best for me in my experience.