Mastery cap? How high do u go?

I know some resto droods start to reforge all spirit into mastery and mastery is making your heals stronger. I am kind of having hard time with this idea. On Heroic Beth, by mid p 2 I go oom. Mainly because the raid in p1 takes crazy damage and a few targets I was assigned to heal (a lock and a boomkin, especially lock) would grab 2 packs of spiderlings and I had to literally pop into a tree form to cast instant regrowth and all my hots to not let him die.

After using an innervate and a domo's trinket I am back at nearly 80% mana but then is what I call "hell breaks loose" - spiderlings running around, people taking insane damage left and right. I spam my rejuvs/wg and keeping my LB with rejuv on lock and occassional regrowth with nourishes but then when we group up on Beth I am like at 2% mana with my innervate on cd.

So here's my question...if I'd reforge my spirit to mastery and get stronger heals, then my heals would heal for better numbers and I would not need to spend lots of mana to constantly cast spells to keep people up?

How high should a mastery be for heroic FL? One drood has 1800 mastery he reforged 2k spirit into it... any suggestions? I haven't tried to reforge all my spirit yet. Just seeking an advice from experienced resto droods. Thx
/bump

Anyone?
There is things your raid can do to prevent some of the damage, for instance...

Kill the pee spiders before they get to their targets... Burn spinners quickly... That eliminates alot of damage right there...

Are you 2 or 3 healing?

I have no problems with mana on that fight, but I always end the fight with about 15-20% mana leftover.
we have usually 6 healers we are a 25 man raid/guild and have 2 teams of 25 people (each) + or - 1/2 who sit outside on a good day when attendance is high.

I did have issues with mana mostly because I was a floating healer at some point and dps was just taking crazy dmg at first. If Innervate is on cd and if u got your 2 pcs t12 I pop into a tree form and cast LB on every single person. In Tree form u can cast LB on multiple targets still. One time on one fight.... I regained full mana bar within seconds it was amazing and my innervate was on cd still.