While I have never raid healed, i do have some suggestions for you.
As stated a few times in this thread, learn to anticapte and set up a mental spell list. If the tank is defficant 20k health and has a hit coming, shield then hasted GH followed by pennace. Some would argue that pennance first, but the GH has a srtong chance to crit, and if it does it will make up the defficany in 1 cast, for 1 casts worth of mana. Healing a melee group and some AoE damage is coming, cause you got the warning from DBM, Shield the squishy rogue, IF, then precast a PoH so that it hits just after the first AoE tick, and then places a Divine Aegis proc on all 5 of them to save them from eating the next tick fully.
BG's are a good place to cut your teeth on large group healing. As well as lerning to keep up over-zealous people...lol
Good luck to you!!