Unless all the clients are on the same system as the application server there will be more network traffic with a FWD implementation than with Progress OpenEdge implementation because of the design of the FWD runtime.
There is one client per ABL client program that is running. This means each interactive user, batch process and Appserver/WebSpeed agent will have a matching client process. There is one socket opened between the application server and each of these client instances. That per-client socket is kept open for the duration of the client's session with the application server. If there are non-ABL applications and/or batch programs that connect to the FWD server, then there is at least one socket open for each of them. Hand coded applications can create more than one session with the server, if they do that, there is 1 socket per session.
If the clients and application server reside on the same system, then there is typically nothing to do to enable connectivity. However, if any clients run on a separate system, then the firewall setup on both the client and the server must be configured to properly support the port(s) in use.
At this time, there isn't a specific equation that can be used to estimate the network traffic, since there are many variable factors. Although the exact amount of traffic is application-specific, ensure high performance for the network connection between the clients and the application server in order to achieve optimal performance.