i think ... lets say that you have used 31 knots of rust. each doing 2500 damage, so...
31 x 2500=(if my math hasnt failed me.)64000
the total damage is one third, thus...
64000 / 3=(this is the real test on my math, ive no calculater!) 31,900 damage
my math is rudimentory at best, so that might be wrong, but i think thats what it is.
Ill let you puzzel out the rest!!
soz, let me correct... damage by knots- 77500
total damage -25833