Free space in NAS storage pool

At the moment only 1 file system has a checkpoint (created at 2GB due to limited space available in the storage pool). We are only able to get successful backups of certain filesystems with TOC using checkpoints. What I want to do is to set the global ndmp parameter so a temporary checkpoint gets created while the backup is running and we do not have to rely on checkpoint schedules. I am working though how much storage I need to add to the pools for each file system temporary checkpoint but what I would like to know if how much "extra" I need available in the pool. I have heard that if there is not enough free space in the pool this can impact the NAS operations but I cannot find any documentation to support that.

I dont think there is a good general number

I really depends how you use the system - like checkpoints, replication and file system extention

there are customers pre-planning and allocating everything upfront and dont leave any space - usually that gets them in trouble when they need some additional small object like a VDM