Here's how it goes - You cast Adlo / Succor on a target and the amount is heals is the same amount of damage that is mitigated by the effect.
Even if the target is at full HP, however, if the spell hits for 255 (just for an example), then there will be 255 damage absorbed by the spell.
So no matter the HP of the target, the number it hits for (even if it can't heal higher than max HP) is the number you'll have mitigated.
Think of it as a low powered cure + stoneskin rolled into one spell.
Because of MP costs it isn't efficient to keep spamming, but if you know there's a big move coming up and see on the focus target bar that it's being charged, it's worth dropping an adlo just for that extra bit of mitigation before you move in to heal what HP has been lost.