I am not sure if there ia any established norm, but think (standardised) TTR, word lenght, sentence length etc are all important indicators of readability. And all of these can be obtained from WordSmtih 3 or 4 Wordlist (statistics). When you can such statistics, use SPSS to compute log-likelihood scores (or chi-square scores), and in cases of very low frequencies, also Fisher's Exact test scores, to see if the diferences between samples are statistically significant. See my posting on statistics for more details.