I just started a course in mathematical statistics. I found an interesting formula that computes the probability of something happening, given the chance of it happening and the number of times the chance occurs.
P(find) = 1-(1-P(drop))^(n)
where P(find) = the probability of finding a Zod
P(drop) = the probability of a Zod drop
n = drops
The drop rates of the Runes (Diablo II) can be found in our wiki and it gave me the drop rate of a Zod rune being 0.00003% chance.
Wich gives: P(drop) = 0.00003/100
n = 10^8 (100 million)
To be 99.999% sure you get a Zod rune you have to get 38 376 412 drops!
But what if you take crafting into account? I wrote this MATLAB file to make an experiment. It's not 100% valid as the random number generator (RNG) in MATLAB isn't a true RNG. It's based on an algorithm and is supposed to give you the same result if you restart the program and do another run. I may have to do another version of this experiment if I find a better way to compute it. (=Have gone to more than 2 lectures in mathematical statistics).