You are very LUCKY to have clients who want you to write to a grade 9 level. I have way too many who think that anything below grade 12 reflects poorly on them!

A mentor once pointed out that once the readability grade level rises above about grade 12, we switch from reading to analyzing, regardless of our education level. That becomes important when writing safety procedures, or those about repairing things that can blow up and kill people. Such writing should allow you to act immediately instead of spending time figuring out what it means, even if only a few seconds. For safety information, we should actually write to a grade 8 level, and to grade 9 for all other.

Some clients are actually beginning to require that we write at grade 9 level.