In my environment, I have kept 10% as the spare capacity per LUN. But there is no best practice configuration or a percentage and it changes depending on the needs of the environment. Our average LUN size is 450 GB which amounts to ~40 GB spare which is not huge. However, if you have 2TB-4TB LUN and you reserve 20% then it would be a huge wastage.
In my opinion if you understand the purpose of the spare capacity you can come with a reasonable and safer no. for your environment. The spare capacity can be used in following situations:
--> To accomodate space for Snapshots. The size required actually depends on the changes being made within the VM. But you can setup alarms in vCenter to control the size of the snapshot.
--> To accomodate space for Virtual Machine's swap file when a vRAM of the VM is increased. The size of the VM swap file is equal to vRAM (without memory reservation configured)
--> To accomodate the need to extend or add new virtual disk on an existing on the Datastore. You can get an average from the no. of such requests you receive on a daily basis.
hope this will assist you in calculating the relevant spare capacity for your environment...
