So you want any traffic trying to leave your external interface to be redirected to your server on its 192.168.1.0 address? That is what that rule says - find any packet on ext_if with the from address set to ext_if's address and change it's to address to 192.168.1.0. I think that will fail spectacularly, as the packets will hit the public network with a non-routeable address, and be summarily dropped by your isp's gateway.
Try this: If you want a computer on the DMZ network (192.168.1.5, say) to handle http: traffic, then this rule accomplishes that:
Code:
rdr on ext_if proto tcp from any to ($ext_if) port http -> 192.168.1.5
A similar rule will be required for each service you want to expose in the DMZ. (This is not really a DMZ, as to properly set one up, you need multiple routable IP addresses to assign to your servers.)