I should mention that any reading level indicator is only a relative measure. What actual grade level the text is and by how much it is more difficult than the next text is highly debatable. However, the relative reading levels should be reliable. For comparison sake, the John Grisham novel that I read a few days ago is rated at a grade 5 reading level. This in part reflects the difference between a teacher supported level and an independent level. Generally we want to, at least, have a Bible at the grade 5 - 6 level or lower and here is why.
The Priniciples of Readability by William H. Dubay. 2004.
Donald Murphy (1947), the editor of Wallace’s Farmer, used a split run with an article written at the 9th-grade level on one run and on at the 6th-grade level on the other run. He found that increasing readability increased readership up of the article 18 percent.
In a second test, he took great care not to change anything except readability, keeping headlines, illustrations, subject matter and the position the same. He found readership increases of 45% for an article on nylon and 60% for an article on corn.
Wilbur Schramm (1947) showed that a readable style contributes to the readers’ perseverance, also called depth or persistence, the tendency to keep reading the text.
Charles E. Swanson (1948) showed that better readability increases reading perseverance as much as 80 percent. He developed an easy version of a story with 131 syllables per 100 words and a hard version with 173 syllables and distributed each to 125 families. A survey of readers taken 30 hours after distribution showed a gain in the easier version over the hard version of 93% of total paragraphs read, 83% in mean number of paragraphs read, and 82% in the number of correspondents reading every paragraph.
Bernard Feld (1948) grouped 101 stories from the Birmingham News into those with high Flesch scores, requiring 9th-grade education or more and those with low scores, requiring less than 9th-grade education. He found readership differences of 20 to 75 percent favoring the low-score versions. Feld’s findings indicated that even a small actual percentage gain for a large-circulation paper greatly increased the number of readers.
That is the kind of study that has influenced translators to reduce the reading level of the Bible. Were they right? You be the judge.