This – but we can get it very wrong, very easily. 5% is always a good option for staying on the safe side.
What we need to know is the following.
Ext4 needs 10% of free disk-space for the defragmentation to work. Hence, the disk which do lots of write operations require it, mostly the OS partition.
For the media-partions, I have 0% reserved space on my NAS, because the medias are usually large files, not small, and I don’t change them very often. And usually my media disks are never full here.
My proposal would be to keep the 5% on the OS partition, and create other partitions for media and static files. That should handle it.
Note that I have a raspberry PI as remote access server which is now running for over 5 years just fine. All static files reside on the / partition, all files that change a lot on an USB stick, and all the rest (tmp file system etc.) on a ram-FS with limited amount of RAM. The USB Stick has a 10% root-reserve, the OS too.
But then again, this is no Media-Server, but my strategy has worked out well