The percentage of African DNA that an African American has is dependent upon what part o the Country the live in. Black people from the South have more African DNA. The further South and East you go the more percentage of African DNA will be present:
"....however, follows a striking geographical trend: African Americans living in Southern states have more African DNA (83 percent) than those living in other areas of the country (80 percent). Conversely, African Americans outside the South have a larger fraction of European DNA. Even within the South, this trend holds: Blacks in Florida and South Carolina have more African DNA than those living in Kentucky and Virginia."