from screenshot my gx has
Str 98 , S.atk 176 Base W.atk 165 Nab's Cloth (Atk4%) and +9 Heroic Backpack (atk+30, critical damage 15%)
Str Bonus = 165*(98/200) = 80.85 , Variance = 0.05*165*3 = 24.75
W.atk = 165+80.85+24.75 = 270.60 rounddown to 270
add 4% from Nab's Cloth = 270*1.04 = 280.8 rounddown to 280
E.atk = 30 add 4% from Nab's Cloth = 30*1.04 = 31.20 rounddown to 31
Atk = 176*2(S.atk*2) + 280(W.atk) + 31(E.atk) + 30(Mastery) = 693
add 20% from Advance Katar Mastery = 693*1.2 = 831.6 rounddown to 831
add 15% from Heroic backpack = 831*1.15 = 955.65 rounddown to 955
minus soft def from target = 955 - 77 = 878
critical damage = 878*1.4 = 1229.2 rounddown to 1229
but real critical damage is 1232!!