The National Center for Education Statistics is a branch of the Federal Department of Education. They released a study in April 2007 on the results from the 2003 National Assessment of Adult Literacy.
There are 3 components of modern literacy testing:
prose, document and quantitative literacy.
Proficiency in prose literacy is 340 to 500 on this test. At 409 you could infer the purpose of an event described in a magazine article.
Proficiency in document literacy is 335 to 500. At 372 you could contrast financial information given in a table regarding the difference between various types of credit cards.
Proficiency in qualitative literacy is 350 to 500. At 400 you could determine the number of units of flooring required to cover a floor in a room, where the area of the floor is not evenly divisible by the units in which the flooring is sold.
In 2003 only 12% of the 19-24 year olds passed the lowest threshold (340) for prose literacy. 13% passed the lowest threshold for document literacy (335). 10% passed the threshold of qualitative literacy (350).