Hello! My code is indeed up on Github (GitHub - ekawaler/AozoraWK) if you’d like to play with it - it was pretty straightforward to put together!
I think if you do some editing in the get_level() function (replace bk_text = aozoracli.client.get_content(id=book_id).text with bk_text=[your sentence here], and change up what the function returns so it prints the max value from kanji_levels instead of returning a percentile), that should give you what you want. You’ll also want to edit the main function so that it just calls get_kanji() and then get_level() once, rather than going through the whole Aozora list.
Take a look and let me know if you have any questions!