clarify the index memory usage analysis

it seems I extracted that data from [this mailing list post][] which
in turn takes it from [this github comment][].

 [this mailing list post]: http://librelist.com/browser/attic/2014/5/6/questions-and-suggestions-about-inner-working-of-attic/
 [this github comment]: https://github.com/jborg/attic/issues/26#issuecomment-35439254

docs/internals.rst

@@ -213,7 +213,7 @@ Indexes memory usage
 --------------------
 
 Here is the estimated memory usage of |project_name| when using those
-indexes:
+indexes.
 
 Repository index
   40 bytes x N ~ 200MB (If a remote repository is
@@ -225,6 +225,9 @@ Chunk lookup index
 File chunk cache
   probably 80-100 bytes x N ~ 400MB
 
+In the above we assume 350GB of data that we divide on an average 64KB
+chunk size, so N is around 5.3 million.
+
 Encryption
 ----------