Hi Bobcheese, the split really depends on the environment, the 80/20 split was an example of a split scope that is typically used when two DHCP servers are located on the network. The DHCP services will normally work in tandem, with no one service being the primary or secondary.
The DHCP scopes will normally be planned so that if you do have 200 agents, then a /23 network can be introduced to cater for the split.
Mitel-user, Mitel are correct in saying this set-up is not really done well on the MCD, as it is only ment to be used for small environments. the problem with having two DHCP servers running the same addresses is that when a DHCP request is sent back to the DHCP servers, based on the Transaction ID field in the request, servers are informed whose offer the client has accepted. When other DHCP servers receive this message, they withdraw any offers that they might have made to the client and return the offered address to the pool of available addresses, so the same IP address becomes available on the 2nd DHCP server for re-distribution.
However, typically, a client may use the Address Resolution Protocol (ARP) to prevent IP conflicts caused by overlapping address pools of DHCP servers, but I don't think the IP phones have this.
You may have to look at increasing the scope.
It may help to know how many end devices we are dealing with here and the subnet class, so I can best help you with a configuration.