Right. Back forty is more of a saying these days than a literal interpretation. But making everything literal is just as neat.
'The back forty' is a beloved American idiom. In the US, 1 township is 36 square miles (6 miles by 6 miles). Each square mile (a section of land) is divided into 16 subdivisions of 40 acres (1/4 mile by 1/4 mile)-- so a quarter section is 160 acres, a common unit of ownership in the early days of farming. The quarter-section farmer would own two 'front 40s' and two 'back 40s'. After the Civil War, former slaves were by some accounts offered reparations consisting of 'forty acres and a mule'." http://www.phrases.org.uk/bulletin_board/33/messages/1130.html