A reader's comprehension sweet spot ranges from 65% to 80%. Reading in this range is challenging enough that you are encountering new vocabulary and sentence structures, but easy enough that you will be able to understand the content of the text at a comfortable reading speed.
Reading outside of this range is fine too. But know that when reading books with a forecasted comprehension rate below this range, a reader might encounter enough unknown vocabulary and difficult sentences that he or she may disengage with the book. Also, when reading books with a rate above this range, a reader is likely to find the reading so easy that opportunities to learn new words or encounter new kinds of sentences might not occur very frequently.
Click here to read more about how to support a reader with books above or below his or her Lexile range.