How many words should you look up when reading in a second language?
Advice usually arrives as a percentage: know 95% or 98% of the words and read for flow. It is a reasonable heuristic and it hides the real variable, which is not how many words you do not know but how much each lookup costs.
If a lookup takes fifteen seconds, even 5% unknown words is unbearable and you will start skipping them. If it takes two seconds, a quarter of the page is workable and you will actually learn the words rather than guessing past them. Readers who describe successfully reading well above their level almost always mention a fast lookup somewhere in the story.
So the practical order is: make lookups cheap, then choose the hardest book you enjoy. On paper that means not typing — Booktionary is built for this, holding the phone over the word rather than switching apps. On an e-reader the built-in dictionary already does it.
The one case for looking up less is when you are reading for pleasure and the plot matters more than the vocabulary. Then guess freely and go on.