I believe that, when we're talking about the reading level publications target, that's typically evaluated using a linear combination of the average words per sentence, and the average syllables per word.

https://en.wikipedia.org/wiki/Flesch%E2%80%93Kincaid_readabi...

If so, then the difference between 8th and 6th grade level works out to about 0.17 syllables per word, or about 5 words per sentence, or some blend of the two. So the gist would seem to be, don't be like me; use periods instead of commas and semicolons, and don't use a word like "orthography" when a word like "spelling" will do.

I doubt that that this is what the department of education was looking at when they assessed adult prose literacy, though. But I'm having a bugger of a time tracking that down, too. I'm not sure that I agree with Wikipedia that a Forbes article that doesn't properly cite its own source is an acceptable citation for that figure.