Using computer simulations, I deduce two very simple rules that can help pin down the keylength of periodic ciphers - a vital first step towards breaking them. Specifically, I find that the period P can be estimated via
P = 66.2 - 1.3H and P = 36.3 - 0.79D
where H is the sum of the percentage frequencies of the seven most common cipher characters, while D is the difference in the sum of percentage frequencies of the of the seven most and seven least common characters.