Shroud of Saint: Use SoS based on %MP left will work for easy leisure dungeon runs, old raid contents and troll tower triology. I recommend using SoS based on mana burning rate and situations. A few examples:
Happy holy spamming sessions or healing intense fights – shroud after casting the first holy or first two cures even when more than 90% MP. Because mp consumption rate in these situations is faster than shroud’s mp restoration rate.

Short boss fights – I don’t shroud when boss has less than 20% HP and I have about 60%+ MP (unless there’s a suicidal dps or boss is going to do more intense damages near the end).

Tough long boss fights (raids for just barely geared w/o echo bonus) – shrouds and ether potions need to be timed and used strategically so that bard doesn’t have to sing ballad in the middle of intense dps check component of fights.