In any case, to me there’s a difference between words I might not have known (or may have seen before but not retained) but understand from context almost immediately, and words that I don’t know and can’t figure out from context. So a sentence with three words that baffle me is a definite signal that a book isn’t within my fluent reading level, but if I can figure words out more or less subconsciously using the information I already have, I’m not all that troubled by them.

If the whole book was like that, it might have been a different story, but really the only guidelines I care much about are the three principles I follow. There are times when three unknown words in a sentence are a sign of a problem and times when they’re not; the tests, for me, are “how much am I understanding, overall?” and “is the book too hard or too easy to enjoy?”