The lease time is a very important factor here.
If you set the lease time to one hour, you know your clients will start to release there IP’s at maximum one hour. If one server goes down, and the remaining server can’t hande out more than 20% of the IP’s needed. You will know that only 20% will recieve a IP address when the lease time has expired.
Now, by default the IP lease time is set to 7 days, which gives you a couple of days to repair the failing server should it go down.
One thing to bear in mind with reducing the lease time to one hour though, is that it’ll generate loads of Broadcast traffic and also the default lease time is 8 days.