How much free space should I leave on a DATA partition?

Data drives or partitions don't need defragging because the operating system only reads from and writes to the drive. Even if a file is read, worked on , and increases in size, it may not fit into its original space, but fragmentation will be almost non-existent.

What happens on the C drive is fast, furious and there are a lot of dynamics going on there, the page file being a major player. Windows needs space to move stuff around, and 25% free space is good.

I think that the 15% free space before Defragmenter would run has been dropped, but it is not a good idea to have less than 15% because Windows likes free space..  

As far as I am aware, the Windows hard drive defect detection mechanism, of which CHKDSK is a part (the big gun), which allows it to deal with small scale errors more or less on the fly. If the problem escalates, CHKDSK is invoked, and Windows shuts down and restarts to give CHKDSK a better shot at fixing the issue without distractions.

If you are lucky, CHKDSK will flag the broken clusters as dead, move data away from the dead zone, and Windows will carry on as normal. If the drive is full to the top, none of the above can or will happen, there being no room for errors of any kind.

So, we leave free space on the drive.. A data drive may not get the rigorous use that the C drive gets, but it is still a dynamic environment where files are read and re-written on demand.

A few bad clusters on a drive is no big deal, and the drive will work well enough. The trouble starts in earnest when clusters start to fail on a regular basis, and you will get warning of this because Windows will appear to hang when reading files. If you ignore this warning, CHKDSK will eventually be invoked every time that the computer is booted.

So, no, you don't have to rush out and buy a new hard drive just because a few bad clusters have been detected.

Does greater free space reduce data loss? Only in as much as Windows has somewhere reliable enough to re-write files that had been a real struggle to read off less than good clusters. In this way, it also buys you time to make regular backups until such time as you can make provision to replace the drive

Even though the 15% limitation has been removed from Defragmenter, you should still leave at least 15% free especially if you store large file sizes, but that is only a recommendation, not a hard and fast rule. Personally, I think that it was a mistake to remove the limit as it just encourages computer users to fill drives to absolute capacity believing that all will be well.