dmg reduction math?

so if I go support, use the mastery Bond of Stone along with , considering that my adc goes with and hit the target dealing the dmg , and that I use my ult and he/she stands in it, how does the dmg reduction math work (not considering armor/mr)? is it additive ((dmg taken % before reduction)-(20+12%=32% dmg taken after ult reduction))-((%dmg taken)x(12+6%=18% dmg redirected))=55.76% of total dmg taken is actually dealt to the adc or is it multiplicative ((dmg taken % before reduction)-20%)-((%dmg taken after ult reduction)x12%)-((%dmg taken)x(12+6%=18% dmg redirected))=57.728% of total dmg taken is actually dealt to the adc

also I'm using ''x''instead of ''*'' because 2(multiplication symbol) causes an italic

sorry if my math are confusing, thx for answering