As disc you should have penance macroed to auto target enemies and never use it for the heal.
PW:S is your only single target heal, Prayer of healing for spirit shell and aoe healing. Archangled macro with divine star or whatever T90 you use. Rest of the time you spam holy fire on cd and then smite.
Prayer of mending is optional and only use it if you are a tryhard...it wont do much. On immerseus you can use flash heal but otherwise never use it. You can unlearn Heal, Renew, Greater heal and pretty much everything that has "Heal" in it's name you have PW:S for that and it's about 10x stronger,
If someone needs single target more than once 15 sec they are probably being a retard so just let them die if your atonement can't keep them up.
If you want a breakdown of what you should be doing on a fight like Dark shamans here's a kill I did a few weeks ago
67 Pennance (each hit counts as a cast)
48 smites
31 PW:S
18 holy fires
15 divine stars (on heroic we stack up in a range group otherwise go cascade)
12 prayer of healing
4 mindbender
and more cooldowns.
On this fight I was top by about 80k hps and ranked in the 80th percentile disc is my alt and I think this char is around 572, bosses were stacked.
http://www.warcraftlogs.com/reports/...2&type=healing log