Drive is almost full

My My Book 3 TB drive is almost full (190 GB free space left).

What should be a “safe” amount of space to leave unwritten before going to another drive?

googling the question … “recommended free space on external hard drive”

results in a lot of different answers … answers seem to be 10% to 15%

but 10% ~ 300GB (of a 3TB) seems like an awful lot !

Thanks, Joey, for your help.

However, I think that I didn’t state my question properly:
It should have, also, said, that I use my WD My Book for backup/storage only and not for anything else.

I do realize more space is needed to be left open for defragmenting, etc., but I have no need to do this.