Max Hp Spell

Max Hp Spell

Ive been messin round with it, just cant find it i guess... But how would you make a spell that when it hits, it does damage equal to a percent of targets max hp?

Some Spell
    Events
        Unit - A unit Starts the effect of an ability
    Conditions
        (Ability being cast) Equal to "some spell"
    Actions
        Unit - Cause (Casting unit) to damage (Target unit of ability being cast), dealing ((Max life of (Target unit of ability being cast)) x 0.20) damage of attack type Spells and damage type Normal

Just do the math. 0.20 x Max life is 1/5 of max life, 0.50 is 1/2, 0.10 is 1/10 etc.