I honestly just sort of play it by ear. If Aspected Helios doesn't hit someone, so long as they aren't around 70%-79% health, I don't worry about it. I'll just walk over to them or cast Regen on them. But if a massive AoE is about to out, I consider using Synastry or quickly throwing a Benefic II their way, depending on the severity of the incoming attack. If an AoE hits before I can get them out of the 70s, I'll use Essential Dignity to get them back up to where the party's health is before throwing out AoE heals.
As for MP management, the advice someone gave me was to use Luminiferous Aether at around 80% MP. That way, it'll be back up again when you need it. I follow this advice more when I'm DPSing in Cleric Stance or I know that I'll have to pump out a lot of healing soon.