I'm I'm down to about 25gb on my OS partition, then I start cleaning up and uninstalling old stuff.
if its my backup drives, i defrag regularly, fill them to near capacity, then replace.and that's usually around 2-5GB remaining. I have no reason not to fill them to capacity.
Once I drop below 1TB, I start evaluating my usage and considering upgrading or expanding...
Or I see if I can relocate and balance out data across storage servers and delete all the temporary stuff I had...
I'll put a plan together and figure out how to increase my storage in a cost efficient manor...
Once it drops below 500GB, I'll start picking up new drives or parts as needed...
I like having some space on hand for moving around data...
On my storage array I use as much as I can until it doesn't let me access the partition. I think that's around 36k.
On my desktop, somewhere around 200MB. I don't usually start deleting stuff or buy more hard drives until I actually *need* the space, which means I end up using 90%+ space most of the time.
Maintaining 20-25% free space is my rule of thumb, especially for raid arrays. There's also the performance consideration for spinning disks: 75% into the surface of the disk, sequential I/O tails off significantly (obviously because the disk is smaller as you move inward).