Review feedback

Author: mungitoperrito

source/faq.txt

@@ -15,7 +15,8 @@ Does ``mongosync`` have to use the same operating system as MongoDB Enterprise S
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 When ``mongosync`` runs on its own hardware, it does not have to use
-the same operation system (OS) as the source or destination clusters.
+the same operation system (OS) as the source or destination clusters it
+is connecting.
 
 Should I increase the size of the oplog in the source cluster?
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -27,7 +28,16 @@ of the replica set :term:`oplog` in the source cluster.
 The ``oplog`` in the source cluster must be large enough to track
 events that happen during the time it takes to complete the initial
 sync to the destination cluster.
- 
+
+The proper ``oplog`` size depends on system hardware, network speed,
+and other factors including system workload. However, assuming network
+transfer speeds of 30-50GB per hour, a rough formula to estimate the
+required ``oplog`` size is:
+
+.. code-block:: shell
+
+   minimumRetentionHours = dataSizeInGB / 30
+
 To learn more about how to increase the size of the ``oplog``, see:
 :ref:`tutorial-change-oplog-size`.