If you want to work out percentages, what you *really* want to be doing is to randomly pick valid usernames and go from there.

The code to randomly generate a username is left as an exercise for the reader. (minus 3 points for saying "Use the loop above to generate an array and then pick random elements from that", although I do grant that is how a mathematician would approach the problem.)

Q: How does a mathematician boil an empty kettle?
A: S/He fills the kettle, switches it on and waits for it to boil.

Q: How does a mathematician boil a kettle full of water?
A: S/He empties the kettle, reducing it to the previous problem.

Disclaimer: I may have studied mathematics in the past...