I thought about this and was wondering whether anyone knows the answer. From my perception, though, I believe Nigerians themselves overestimate the number of educated Nigerians because Nigeria has many non-urban areas, places in which lack (or have inept) educational infrastructures and, therefore, harbor many children who do not have the opportunity go to school.

Well you know Nigerians government officials alwys forge figures on literacy levels to the organisations abroad so as to get funding,but depening on you defination of literacy.The educational sysytem is bad and degrees put food on the table ,so some Languages:
English (official), Hausa, Yoruba, Igbo (Ibo), Fulani

Literacy:
definition: age 15 and over can read and write
total population: 68%
male: 75.7%
female: 60.6% (2003 est.)

School life expectancy (primary to tertiary education):
total: 8 years
male: 9 years
female: 7 years (2004)

Education expenditures:
0.9% of GDP (1991)
country comparison to the world: 180
people have certicates but they are still ignorant in Nigeria How ever visit CIA.com

I would think the definition of being literate is graduating from secondary school (high school).

That is not true ,in Nigeria a litrate person is somebody that can read and write.