Hi Glenn, I'm not aware of a document that defines CGI better than the RFC and I don't know it enough to disgress from the published standard (even if it is not what isdone today as I don't know the current practices enough).

Here is the variables defined by nginx 1.10.3 on Debian:

    fastcgi_param  QUERY_STRING       $query_string;
    fastcgi_param  REQUEST_METHOD     $request_method;
    fastcgi_param  CONTENT_TYPE       $content_type;
    fastcgi_param  CONTENT_LENGTH     $content_length;

    fastcgi_param  SCRIPT_NAME        $fastcgi_script_name;
    fastcgi_param  REQUEST_URI        $request_uri;
    fastcgi_param  DOCUMENT_URI       $document_uri;
    fastcgi_param  DOCUMENT_ROOT      $document_root;
    fastcgi_param  SERVER_PROTOCOL    $server_protocol;
    fastcgi_param  REQUEST_SCHEME     $scheme;
    fastcgi_param  HTTPS              $https if_not_empty;

    fastcgi_param  GATEWAY_INTERFACE  CGI/1.1;
    fastcgi_param  SERVER_SOFTWARE    nginx/$nginx_version;

    fastcgi_param  REMOTE_ADDR        $remote_addr;
    fastcgi_param  REMOTE_PORT        $remote_port;
    fastcgi_param  SERVER_ADDR        $server_addr;
    fastcgi_param  SERVER_PORT        $server_port;
    fastcgi_param  SERVER_NAME        $server_name;

    # PHP only, required if PHP was built with --enable-force-cgi-redirect
    fastcgi_param  REDIRECT_STATUS    200;

Someone that knows CGI better than me may know the way forward