I always just keep BoL up on the tank, since he starts to take heavy dmg from the Hideous once he's collected all 9 bloods, especially if he takes them too soon by mistake, yet I never go oom on spine. I have 3165 unbuffed spirit, and I hit 4050 spirit with my normal mode spine trinket (+880 spirit). I still push out
roughly 25k hps, whilst always staying above 90% mana. I don't start to lose mana until on the last plate, when the other 1-2 healer(s) are oom, and I need to practically solo heal (this is when all healer CD's + Divine Light = Awesome, because 1-2 divine lights remove the debuff, depending on the other healers mana)
Thinking about reforging 880 spirit into haste, since I never had problems healing before I got the 880 spirit trinket, and I don't like Seal of the Seven Signs normal mode ^^ (it procced once in a 2 minute test I did) also, noticed that all my gear is reforged to spirit, so I will just have to be full mana at all times
(401 Holy Paladin, Ragnaros)