From a newsletter stylebook:
“How to calculate readability:
Count the total number of words in the story.
Count the total number of words with less than five letters.
If 70% of the copy has words of five letters or less, it is very readable!”
Italics mine. Also mine is the four-letter word expectorated (read: spit out) after absorbing this advice. I guess it works OK if you’re a jar of pasta sauce or a stop sign, though.