Damage calculation help

I am assuming these formulas:

Dmg reduction = arm / (arm + 100 )

0 armor pen. No masteries. No runes.

I created a new game with just me and Chogath Bot. I am Ashe with 57 atk. I hit Chogath twice consecutively with 32 armor. I deal 37 dmg then 41 dmg.

I find my calculator and the dmg reduction from chogath should be 24.2%. Effectively I do 75.75% of my total damage to him.

57* 75.75% is 43 damage. With Ashe's passive it should be 47.5 or 47 dmg.

What am I doing wrong?