How to set up outbound NAT?

I have successfully set up Destination NAT (DNAT) so I can receive incoming traffic from an "additional ip address" (.11) in to one of my LAN machines. However, when this machine goes out to the internet, it is still using the primary interface ip address (.10).

I am sure I have to set up some kind of NAT entry to map the internal machine's address/traffic out to the secondary static internet address (.11). I tried adding a SNAT entry, specifying my LAN machine as the traffic selector / ANY / ANY, then source translation of my external WAN address (.11).

However, when I go to a whatismyip site, I am still seeing .10, which is the primary interface ip / masquerade entry.

What am I missing, or is there already a good write-up on this?

Thanks!