The equation is:
P = (1-pow((1-p), n)) * 100 [%], where n means ammount of kills and p is probability of the drop (in this case 1/5000)
Just out of curiousity, what is "pow"? I'm not very familiar with the english mathematical terms.
power, for example pow(2, 5) means 2^5
http://puu.sh/84jqm.png