Pia Savage points out in her comment on the previous post that the “score” in those readability tests comes most likely from the Flesch-Kincaid Readability Test. This has been around since John Howard was in short pants; we learned about it, and similar measures, in Dip Ed in 1964, and they were venerable then. The grade level is calculated with the following formula:
Obviously one cannot “fail” such a test, nor is it in itself a good or bad thing to get a particular result as the test totally ignores both meaning and context. That is a serious failing. Nonetheless, it is a useful indicator of the likelihood of communicating effectively.
I am however something of a fan of the Plain Language movement. I spent a very profitable year in 1978 studying Language Variation and Stylistics with Professor R D Eagleson, one of the Australian gurus of that movement. I attach his Writing in Plain English.