Formula for weapon damage.
https://www.wolframalpha.com/input/?i=x+%3D+(100+-+y+%2F+(y+%2B+400)+*+90)+%2F+100+*+z+-+r),+y+%3D+1000,+z+%3D+60,+r+%3D+1
x = (100 - y / (y + 400) * 90) / 100 * z - r), y = 1000, z = 60, r = 1

x - final damage
y - items defence
z - initial damage
r - vitality defence