I really don't see anything wrong with it but, I would write it like this:
iptables -A INPUT -i $EXT_NIC -p tcp --dport 22
-m state --state NEW -j ACCEPT
iptables -A INPUT -o $EXT_NIC -p tcp --sport 22
-m state --state ESTABLISHED,RELATED -j ACCEPT
Let me know if this helped, hope it...