Configuring PPTP

Using a dedicated NT 4 server with ConSeal firewall and WinProxy (Ositis) proxy server. We are now in the process of setting up RAS / PPTP on this server - behind the firewall - and have enabled GRE (47) and port 1723. Have followed Microsoft's install procedure and completed up to IP Forwarding. We're unclear on the routing details, and whether registry changes are required, when PPTP server is "behind" the firewall. What is necessary to finalize our configuration.
Appreciate your help, thanks.