1% would be fine... Else one day when they e.g., want to upgrade from ext4 to ext5 etc. and face http://www.debian-administration.org/article/643/Migrating_a_live_system_from_ext3_to_ext4_filesystem and end up with Kernel panic – not syncing Attempted to kill init when something goes wrong. All because they had no more room to copy /sbin etc.