The application is for a Bluesound Node (latest) storage and not for a PC.
You can put 1.81 TB on it, there is no headroom needed. No damage is done if you run out of space to put new music on it. And it does not make the SSD unstable. The only reason you need to leave a little headroom is for PC apps and programs as they sometimes need some disk space to create temp files while they run. If it makes you feel more comfortable, you can leave 10-20% empty as ghd stated, but it’s not necessary.
I had a bluesound vault, and now use an Aurender N200 with two SSDs, just fyi. My 2TB ssd is full, the 4TB is about 50%, the Aurender has a music cashe built into it. Reading about bluesound node on their website, it only cashe’s artwork, not music.
The only reason for more headroom is limited number of "writes" to SSD. Controller from time to time rotates data location to avoid any cell being written too many times. Disk with large overhead will last longer, but it applies to extreme conditions (traffic + temperature) and many years of use, if not decades. At this point people buy new gear anyway. I doesn't even apply to audio, since 99.9% of time there will be "reads" only.