One other thing you could consider if you want to prevent people guessing your codes is to add a checksum character into the code. At its simplest this can just be the character that might represent the sum of all the other characters. I'm sure you could work out the details of how that would work pretty quickly.
One other thing you could consider if you want to prevent people guessing your codes is to add a checksum character into the code. At its simplest this can just be the character that might represent the sum of all the other characters. I'm sure you could work out the details of how that would work pretty quickly.