Nobody measured it recently. 300 is a round number that has been copied from tool to tool for years, and once enough calculators agreed on it, agreeing became the reason to use it. It is convention, not evidence.
The evidence is newer and less convenient. Brysbaert's 2019 review pulled together 190 studies of reading rate and found silent reading of English prose running at about 238 words a minute for non-fiction and 260 for fiction. Both are well under the assumption, which means every estimate built on 300 tells you a book is shorter than it is.
We use 250 across this site. It sits between the two figures, it is slower than what you will see elsewhere, and it is closer to how people actually read. We would rather be the site that says a book takes longer and turns out to be right.