Is there any point in tattooing it on the back of the hands of people who don't read?

That said, https://readabilityformulas.com/readability-scoring-system.p... is a good sanity check. Over half of people can't read text above a grade 8 readability. Worse yet, we aren't conscious of the effort that reading takes. Not until we are struggling. So competent people have little sense of the barriers they create.