Finch's landing which is indeed 20 miles west of Maycomb is the family homestead of Atticus Finch.  It is the Finch families only real claim to respectability in the Southern society in which they live.  As described in chapter 1, it was founded by their ancestor, Simaon Finch, who had come from Cornwall, England and it was a cotton growing concern.  Atticus was unusal for a Finch male in that he left Finch's Landing to make his living at the Law in Maycomb.  As the story develps we will see that Atticus is an unusual man in other regards as well.

Finch's Landing is twenty miles west of Maycomb. 

In Chapter 13, Maycomb is identified as being twenty miles east of Finch's Landing, so by working backwards, we can figure out that conversely, Finch's Landing is twenty miles west of Maycomb.