Grade level = 0.06 XI + 9.55 Xz + 6.97 Xs + 2.69, where
XI =average sentence length in words
Xz = ratio of prepositional phrases to number of words
Xg = ratio of non-Dale list words to number of words

For the list of 769 words http://www.betterendings.org/homeschool/words/769words1.htm
For historical reference, not a priority.