Looks correct. Your rule says to forward packets destined to your PPPOE ip on port 223 to go to 172.16.120.20:80
In Winbox and Webfig in IP > Firewall you can view the packet statistics, every time a connection comes in, the packet count goes up.
From:
http://wiki.mikrotik.com/wiki/Forwardin ... nternal_IP
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Forwarding a port to an internal IP
This example will show you how to forward port (tcp 5900) to an internal IP using destination NAT. 69.69.69.69 is the example wan IP, 192.168.1.101 is the desired internal destination.
/ip firewall nat add chain=dstnat dst-address=69.69.69.69 protocol=tcp dst-port=5900 action=dst-nat to-addresses=192.168.1.101 to-ports=5900
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