300m / 300k largely just simplifies the maths. That's useful for very rough napkin calculations.
Going beyond that: for starters you can exclude children to a certain age (say 5, 10, 15 years), based on limited literacy, and adults in later years with visual and cognitive deficiencies (glaucoma, macular degeneration, dementia, other cognitive conditions). Ten-and-unders alone are about 10% of the total population: <https://www.neilsberg.com/insights/united-states-population-...>.
Then there's actual measured adult literacy rates which are ... far more sobering than you might think. At least half the U.S. adult population would struggle strongly with any modestly complex text, fiction or nonfiction:
My sense is that it's more of an "it is what it is" situation. That is, if you're operating in a domain which requires or presumes literacy, then you'll do better to have a realistic appraisal of what the reality is.
Among other factors, the level seems to be relatively consistent over time, it corresponds to other similarly nuanced measures (the OECD computer literacy survey mentioned in my linked 2021 comment, Jean Piaget's work on intellectual attainment levels, presumably based on 1950s/1960s France), and other broad measures.
The US has a strong sense of the actual literacy situation because it actually tests for this, where many other countries apparently do not, or don't publish their findings. "Highly literate" is a pride and prestige factor for many countries, and rates of 95--99% literacy (often given) likely are based on very low minimum standards.
I also suspect that there may be some negative consideration given the large immigrant / non-native-English speaking population in the US (where some of the latter is in fact native-born but in insular communities), where individuals may have literate capabilities in their native language but not English. Given that the lowest rates of adult literary attainment are in southern border communities (most notably in the Big Bend region of Texas) this seems at least possible.
If you are highly literate and technical you're all but certainly an outlier amongst the general population, and your own immediate experience and that of those you encounter most often is probably not a generalisable one.
In the technical context I've called this the Tyranny of the Minimum Viable User, which addresses both the fact that widely-used computer interfaces must be exceedingly basic (to avoid disenfranchising the vast majority of the population) and that this means that proficient or expert users face challenges in trying to address their own complex needs on such systems unless there are ready means of extending the system capabilities to match their personal ability and needs. The tension here is absolutely innate and inevitable.
Also, if you're trying to sell books, you're selling into roughly 10--20% of the population at best, most of the time. Which is why other forms of media (music, video, games) tend to be so much more popular, in all senses of that word.
That's an incredibly large number of illiterates. Am surprised ...