How much black was Amanda Starr?

In the movie, upon the death of Amanda Starr's father it is revealed that her wealthy plantation owning father was heavily in debt and more, her own mother was part black. All her life, Amanda Starr easily looked 100% Caucasian, so this begs the question, how much black was the Amanda Starr character?

The movie says her mother was black but if so, Amanda would have been a mulatto, easily discerned. Here's my own theory. Amanda's mother was a slave as the movie stated but no more than 1/4th African, making her what was called back then, a 'quadroon'. That would make Amanda Starr, 1/8th African, called back then an, 'octoroon'. It seems Amanda's mother may have been a, 'bought' white woman who had some African heritage who became Mr. Starr's mistress. Back then, white men were capable of falling in love with their less than 100% Caucasian mistresses but were prevented by law and social custom from marrying a white woman who possessed African blood. The mother that Amanda Starr thought was hers wasn't.