Life Drain changing

I'm trying to create a simple ability that deals a very large amount of damage over time to a unit, and as the level increases, a larger portion of that mana and health damage is returned to the caster. How do I make it so only a % of the damage done via life drain is returned to caster?

You can't change life drain to drain % of target's hp (as far as I know)

You will have to trigger the whole thing.

Not to drain % of targets hp, but to have the spell do x amount of damage, but give the caster only 50% of x.

If the spell can't do it itself, you'll need to trigger it.

Yes, I know that. The problem is that the spell has many options like additional life decayed and life transferred, both of which are unused and that I have no idea what they do.

Then test it. Duh.

I'm pretty sure they relate to the functionality added (via a patch) to Mana Drain to allow one to A) Drain over your max mana with a resultant decay of the extra and B) Transfer excess to another friendly unit.

I have a nagging suspicion that these two functions don't work with Life Drain, but i may be misremembering.