How much ovehead is needed on the LUN for VMware snapshot based backup

Hi Guys,

We had a problem where one of the VM's LUN got filled up with leftover snapshot that netbackup could not remove causing the VM to shut down. In order to mitigate the issue we have decided to allocate an overhead of 20% to the LUN size. Is 20% good enough or should we allocate more space. I mean is there a recommended %.

Master: Netbackup 7.5.0.6 on Windows 2008 R2 Enterprise SP1 64 Bit

Thanks