Ignore defense rate is calculated as: % of damage that can be hit =100%- (Boss defense %* (1- Your Ignore defense rate %))

e.g.

84% Ignore defense, play 200% defense boss (Advanced Belem) : 100%- (200% (1-84%)) =68%

52% Ignore defense, play 200% defense boss (Advanced Belem) : 100%- (200% (1-52%)) =4%

67% ignore defense, hit 300% defense bosses (Siu, Damian, Lucid, Will) : 100%- (300% x (1-67%)) =1%