Understanding Recoup

You recoup #% of damage taken from hits over 4 seconds. This is increased by increases/decreases to regeneration.

If I Recoup 20% damage as mana and have 400% increased mana regeneration rate, then I regenerate 100% of damage taken from hits over 4 seconds.

If I have The Agnostic then it becomes 80% to mana and 20% to life.

If I have a further 30% life Recoup, then 80% damage is Recouped as mana and 50% is Recouped as life.

Am I understanding all of this correctly? If I had a 60% MoM then I would net a positive 10% life and 20% mana. I could then create a Heartbound Loop setup with reliable sustain.

Regeneration can't affect how much source you recoup, maybe you're confusing regeneration with recovery rate.

I don't really wanna get into this interaction, but Agnostic behaviour entirely depends on how many life you lose. In practice you can easily lose life equals to 50% of your mana, so you'll recover it via Agnostic over 2.5 seconds. And vise versa you can take very small part of life from the ring's damage and Agnostic will drain less mana to recover that life.