Cisco 501 PIX port forwarding (outside DHCP)

Hello,

I am trying to switch from a Cyberguard 570 to a Cisco 501 PIX. The PIX I have read about creating a forwarding rule but they all centered around a static external IP. IE something like this: static (inside,outside) tcp netmask 0 0

however how would I create this rule with a DHCP assigned external IP address?

Also the access-list would need to be created as well from what I understand...would that be something like this?

access-list

or in my case specifically

access-list torrents permit tcp

I guess overall my question is how to identify a DHCP assigned external interface IP address in these rules.