To recap, you should perform the following tasks when designing replication for your directory service:
Create a map of your organization's physical network.
Inventory your existing and planned directory-enabled applications, and understand the load that they will generate.
Note on the map the locations of your directory-enabled applications.
Locate at least one replica within each network location connected via a slower and/or unreliable network link.
Add additional replicas to handle the client load. Optionally, create dedicated replicas for directory-enabled applications that make extensive use of the directory.