I think the most important is your interest in this particular text/lesson.
For example, if I find the topic of the new text interesting, I will read it, whatever the percentage of unknown words is.
It's got to be really interesting if I'm to struggle through a text where there's more than two words on every line that I really don't understand.
In addition, I don't really believe the percentage of new words. Maybe many known words or the ones that should be ignored, for example citizens' names, cities' names, verb conjugations, plural forms, "...'s", typos, foreign language words, (In Chinese texts, there are many mis-combined words on LingQ) etc are hidden in texts. Unless we know certain proper nouns and others, we can follow texts.
I am guided largely by my interest in the subject in selecting what to study. However, sometimes I will search for easier content to study just to vary things a bit.
I feel great satisfaction when I see the % new words going down, for items that I import from the same source.
The % new words is a good indicator to me of how much easier these contents are becoming, in other words of my progress in the language, despite the fact that the "new words" may include names, or forms of words that I already know.
New Testament simplified, Matthew chapter 3
New words 56 (28%)
(This text is quite short.)
In fact, 26 mis-combined words are found in it, and I clicked "Ignore" on all the words. Then, the real percentage of new words is 15 %. However, in these new words, there are some words I've ever encountered before in some books outside of LingQ.
I also tried to see some French texts considered difficult on LingQ. I am surprised that all the texts include less than 20% new words, in which there are conjugations, names, etc. For example, In Collection "La France pendant la deuxième guerre mondiale, Faut-il arrêter les combats?", each text has 5 %- 14 % new words ( including conjugations, names, etc).
To me, not difficult? I don't think so (because there are many yellow words and I must have certain basic knowledge of France.)
I must say to my-self that understanding texts is not just translating them into my tongue.