Drain is very rarely used for HP healing. Maybe in an ohshit moment but otherwise I only use it when I'm in that final burn phase, low on MP or just got revived after messing up. At least as SMN. AS SCH I only really use stacks if I'm dpsing and baning (if I'm in cleric I just play like SMN) or when throwing down a soil/lustrate. Except as SCH I try to keep a stack til aether's off CD, while as SMN I just pop a Fester as soon as it's off cd and my dots are up. I'm already doing 650~1100 damage festers and I hardly have any DL.