I found an event called "Unit Damaged" which sounds like a good start.
But now I need a Term to compare the unit health either to a value, variable or percent, something.
I can't seem to find a Term for this purpose.

Is there such a Term?

Most likely you just use ValidateUnit as a term, and select a validator that checks the unit's life. (there are many already, called stuff like Caster Life GT 45, GT is greater than)

Thanks man, found what I wanted