Pay close attention to the part where it says 20% increase for a VM installation. This accounts for VM processing overhead.
Also, VMs sometimes stray from time, so you'll want to make sure that you have NTP actively syncing. We've seen instances where the server time has strayed behind by 2 hours over 3 months time.
These recommendations are also based on using VMWare ESXi. Other VM applications may take up additional overhead.