Kolkata is approximately where St. John went to perform his missionary work in India. This is evidenced in Chapter 34 when St. John states, "'And I shall see it again…in dreams when I sleep by the Ganges'" and in Chapter 35 when Diana states, "'Plain! You? Not at all. You are much too pretty, as well as too good, to be grilled alive in Calcutta."' The Ganges River is vast and extends through much of northern India, but it does intersect with Kolkata specifically, so these two instances of dialogue both support this approximation.

Bronte, Charlotte. Jane Eyre. Service and Patton. 1897. Project Gutenberg, http://www.gutenberg.org/files/1260/1260-h/1260-h.htm.