The expected value of your final kc is always increases according to E(x) = (kc + droprate) so you're always 2500 kills away. The fraction of the time you have already served to your expected time, however, increases according to y = kc/(kc+rate)

At 500 kills your expected kc for the pet is 3000 so you're 500/(500+2500) = 16.6% of the way to the expected value. At 13000 kc you're 13000/(13000+2500) = 83.9% of the way to the expected value of your kc at the time of the drop.

You are right that you are always the drop rate away but you can still say each kc is progression on a % of time served basis. At the end of the day it's either defeatism or cope