With vSphere 4.0, VMware introduced the ability to specify a percentage next to a number of host failures and a designated failover host. This was a single value for CPU and memory. With vSphere 5.0, this has changed as it is now possible to select different percentages for CPU and memory as shown below.
The main advantage of the percentage based Admission Control Policy is that it avoids the commonly experienced slot size issue where values are skewed due to a large reservation.
Some of the other advantages of this Admission control policy are that this is much more accurate as it considers actual reservation per virtual machine to calculate available failover resources. This also means that cluster dynamically adjusts when resources are added.
But wait this has been written about several times, what is special that I am trying to add here, well as we know for each design decision that we make there are certain constraints, for this policy selection you need to manually calculate the % needed when adding additional hosts in a cluster and if you want number of host failures needs to remain unchanged.
To answer this I have simplified the calculation process for the folks going to use this as their preferred HA Admission Control Policy.
Take a look at my HA Admission Control Calculator which solves this for you , all you need to do is to put the number of hosts in the cluster , how many hosts failures you want the cluster to tolerate , rest is calculated and you will be given a single % value for both CPU and memory.
Hope this helps people trying to use the % based Admission control policy make their task easy.