Use the vSphere Performance Profiling dashboard to determine a suitable Service Level Agreement (SLA).

This dashboard displays the performance of all vSphere clusters and their shared datastores. For each VM, the dashboard displays the performance metric at the 99th percentile. For each SLA (CPU, Memory, Disk), the dashboard shows the worst and the average performing VM.
Note: Avoid setting the SLAs at a 100th percentile, as the 100th percentile usually indicates an outlier.