How does the 80/20 rule offer DHCP Redundancy

I've noticed questions in both DNS/BIND/DHCP/WINS Issues and in some of the Server forums about DHCP redundancy. The best rule of thumb I've ever heard of is called the 80/20 rule. Here's how it works. DHCP Server A is set-up with a range of 10.1.1.1 to 10.1.1.254. Excluded are 10.1.1.1 to 10.1.1.203. This leaves a range of 10.1.204 to 10.1.1.254 for Server A to assign to Subnet A, or about 20%. DHCP Server B is configured with a range of 10.1.1.1 to 10.1.1.254, with exclusions of 10.1.1.204 to 10.1.1.254. This leaves Server B a range of 10.1.1.1 to 10.1.1.203 to assign on Subnet B, or around 80%. If Server B goes down, clients on Subnet B can then get an ip from the exlusion range of Server A, and if Server A goes down, users on Subnet A can get ip's from the exclusion range on Server B. Hope this helps.