In 8.0-RELEASE it becomes sys/netinet/ipfw/ip_fw2.c
	I've hacked this, based on ../ip_fw2.c.REL=7.2-RELEASE.diff 
	but not thought the code through.
In 7.3 it goes up a directory & changes syntax

*** 7.3-RELEASE/src/sys/netinet/ip_fw2.c	Fri Sep  3 17:08:02 2010
--- new-generic/src/sys/netinet/ip_fw2.c	Fri Sep  3 17:08:44 2010
***************
*** 151,157 ****
  	u_int32_t		value;
  };
  
! static int autoinc_step = 100; /* bounded to 1..1000 in add_rule() */
  
  extern int ipfw_chg_hook(SYSCTL_HANDLER_ARGS);
  
--- 151,157 ----
  	u_int32_t		value;
  };
  
! static int autoinc_step = 20; /* bounded to 1..1000 in add_rule() */
  
  extern int ipfw_chg_hook(SYSCTL_HANDLER_ARGS);