The only thing wrong is that if you want to heal 5% of a unit's MAX health, your should change the equation. You gotta take the current unit's life and then add the default unit's life x 0.05 (5 % of his max hp).
Good trigger btw :P
it would be something like :
Unit - Set (Picked unit) Life to (((Picked unit) Life (Current)) + (((Picked unit) Life (Default)) * 0.05))