His key was developed by splitting of the alphabet, A through to M and N through to Z. We can see that the 13 Symbol cipher begins with an A and ends with an M. So, what we will do first, is place the alphabet alongside the 13 Symbol cipher and see if we can find any organization or patterns that exist using simple mathematics.
The 340 cipher was the Zodiac Killer's unsolved masterpiece, and being a man with an inflated ego, driven by self publicity, what better way to drop us a hint by placing his signature or pseudonym on the final line of the 340 cipher. You will notice he placed the Zodiac crosshairs symbol, followed by what could be interpreted as Zodiac, and then an upside down Aries symbol (with a line on the top and a dot to the left). So, what we shall do is place the correct spelling of Zodiac alongside the version he placed on the 20th line of the 340 cipher and use the identical technique we used to create the symmetry on the 13 Symbol cipher. The alphabetical difference of ZOD produces three zero's, the IAC produces 888. These two sets of figures meshed together gives us the three circled 8's found on his next cipher, as though he was carrying them forward. It is also apparent on the 13 Symbol cipher, that either side of the three circled 8's are the Zodiac crosshairs and an inverted Aries symbol, almost identical to the final line of the 340 cipher. So, the author has effectively carried eight symbols from one cipher to another.