sorry, your math is is bad

first - equation 1-(100/(X+100)) where X is armor value, is not linear (inverse is true - graph approaches 100% reduction but never reaches it) make graph in excell and u will see

second your example is not valid, because there is no 50% reduction of armor (last whisper reduce 40%) and because funktion is not linear, samples are behaving not linear (50AR - 30AR = 10,2 %difference), (100AR - 60AR = 12,5% difference), (200 AR -120AR = 12,2 %difference) and so on