I ran out of space for all my rules, so reduced spacing from 100 to 20.

I have not read kernel to check if the kernel might now run out of space
somewhere, now I can have more rules,
I guess if its doing a malloc it will be OK.
It's been running fine with me since at least January 2000 with my 688 rules.
I did do a scan of entire /usr/include & /sys for "100" in case
somewhere else a programmer has assumed the same 100 but without using
a common define, that would have been evil, & deserved to be discovered.

Ideally one could convert this 20 to a define & use a sysctl to amend it
in the MIB base of net.inet.ip.fw

4.11 has no ip_fw.c

*** 3.3-and-3.4-and-4.1.1-to-4.10-not-4.11-RELEASE/src/sys/netinet/ip_fw.c	Sun Aug 29 18:29:44 1999
--- new-generic/src/sys/netinet/ip_fw.c	Fri Jan 28 11:17:55 2000
***************
*** 1001,1007 ****
  		return(0);
          }
  
! 	/* If entry number is 0, find highest numbered rule and add 100 */
  	if (ftmp->fw_number == 0) {
  		for (fcp = LIST_FIRST(chainptr); fcp; fcp = LIST_NEXT(fcp, chain)) {
  			if (fcp->rule->fw_number != (u_short)-1)
--- 1001,1007 ----
  		return(0);
          }
  
! 	/* If entry number is 0, find highest numbered rule and add 20 */
  	if (ftmp->fw_number == 0) {
  		for (fcp = LIST_FIRST(chainptr); fcp; fcp = LIST_NEXT(fcp, chain)) {
  			if (fcp->rule->fw_number != (u_short)-1)
# The next patch is for 4.1.1-RELEASE
# ***************
# *** 1009,1016 ****
#   			else
#   				break;
#   		}
# ! 		if (nbr < IPFW_DEFAULT_RULE - 100)
# ! 			nbr += 100;
#   		ftmp->fw_number = nbr;
#   	}
#   
# --- 1009,1016 ----
#   			else
#   				break;
#   		}
# ! 		if (nbr < IPFW_DEFAULT_RULE - 20)
# ! 			nbr += 20;
#   		ftmp->fw_number = nbr;
#   	}
#   
# The next patch is for 4.2-RELEASE
# *** 1009,1016 ****
#   			else
#   				break;
#   		}
# ! 		if (nbr < IPFW_DEFAULT_RULE - 100)
# ! 			nbr += 100;
#   		ftmp->fw_number = frwl->fw_number = nbr;
#   	}
#   
# --- 1009,1016 ----
#   			else
#   				break;
#   		}
# ! 		if (nbr < IPFW_DEFAULT_RULE - 20)
# ! 			nbr += 20;
#   		ftmp->fw_number = frwl->fw_number = nbr;
#   	}
#   
*** 4.5-RELEASE/src/sys/netinet/ip_fw.c	Mon Jan  7 23:40:22 2002
--- new-generic/src/sys/netinet/ip_fw.c	Tue Feb 26 17:33:56 2002
***************
*** 1609,1616 ****
  			else
  				break;
  		}
! 		if (nbr < IPFW_DEFAULT_RULE - 100)
! 			nbr += 100;
  		ftmp->fw_number = rule->fw_number = nbr;
  	}
  
--- 1609,1616 ----
  			else
  				break;
  		}
! 		if (nbr < IPFW_DEFAULT_RULE - 20)
! 			nbr += 20;
  		ftmp->fw_number = rule->fw_number = nbr;
  	}
  
# The next patch is just for obsolete old 3.4, 
# not for 4.1-RELEASE or 4.5-RELEASE
# *** 3.4-RELEASE/src/sbin/ipfw/ipfw.8	Wed Oct 20 15:07:36 1999
# --- jhs/src/sbin/ipfw/ipfw.8	Thu Apr 13 11:25:36 2000
# ***************
# *** 228,234 ****
#   Multiple rules may share the same number and apply in
#   the order in which they were added.
#   .Pp
# ! If a rule is added without a number, it is numbered 100 higher than the highest
#   defined rule number, unless the highest defined rule number is 65435 or
#   greater, in which case new rules are given that same number.
#   .Pp
# --- 228,234 ----
#   Multiple rules may share the same number and apply in
#   the order in which they were added.
#   .Pp
# ! If a rule is added without a number, it is numbered 20 higher than the highest
#   defined rule number, unless the highest defined rule number is 65435 or
#   greater, in which case new rules are given that same number.
#   .Pp
# 
--------------------------------------------------------------------------------
Files with "100" that I probably can ignore, but not certain, & would 
appreciate confirmation from someone.
 sys/netinet/tcp_debug.h		#define	TCP_NDEBUG 100
 sys/kern/kern_sysctl.c			k += 100;
 sys/netinet/ip_divert.c		#define	DIVSNDQ		(65536 + 100)
 sys/netinet/ip_dummynet.c			    if (p->queue_size > 100)
 sys/netipx/spx_debug.h		#define	SPX_NDEBUG 100
 sys/netns/spp_debug.h		#define	SPP_NDEBUG 100