Last I read, Dignity maxes out at 1000 potency when the target is at 1HP, and is about 700 potency when the target is at 50% HP, so if you use it when the target is at half, it's essentially a frinstant Benefic II. The beautiful thing about Dignity is that its cooldown is so short you can use it 4~5 times before Benediction would cool down. If you use it frequently, that frees up 1.5 GCDs per minute that you could spend on DPS or AoE heals. It also frees up about 1000 MP per minute, which should help since you mention MP worries on AST.
Speaking of MP worries, more frequent use of Lightspeed and Synastry will help relieve some stress there. Lightspeed reduces the cost of 4~5 spells by half, and if it happens to fall within your Celestial Opposition (later, lv60 action) it lasts for 9~10 spells. Synastry will also save you MP on heals, as its piggyback heal effect can be used like a stronger version of Largesse if the tank is taking lots of damage, or to spread the healing around without having to take extra actions or spend MP on AoE heals.