they will have a performance hit since the R/W head has to move further out to write data
Sorry, but I have a different opinion there.
First, the dimensions of the platters inside drives have not changed in years. What has changed is the number of platters and how much data the R/W head can cram into the same amount of space (density). So just because today's drives have monster capacities, that does not suggest it takes longer to retrieve the data. In fact, seek times have improved greatly and while rotation speeds have remained the same, the data density is much greater so the R/W head can read in or write much more data per rotation. Thus, access times are greatly improved with larger capacity drives.
And to those seek times, whether reading or writing, note that really only applies to finding the first segment of a file. Unless very heavily fragmented, each following segment will be read from or written in the adjacent storage location. There will be no moving further out.
Also data is NOT saved on hard drives with all the data jammed at one end of the platter. Instead, files are scattered all over the disk for the purpose of keeping all the file segments in sequential order (not fragmented). But over time as those files are modified, they can become fragmented and scattered about even further if there is not sufficient amounts of free disk space. It is this fragmentation that slows down access.
Fortunately, unless you dinked with the default settings, Windows automatically defrags disks weekly (if needed) to minimize these issues.
Also, the R/W does not "park" itself at the beginning or end of the platters after every read or write either. Automatic head parking is only done when the power is removed. This means the distance, thus "seek" time it needs to travel to get to the first file segment for the next read or write is random and immaterial because it is all based on where it performed its last access.
So the only performance hit comes due the fact a shortage of free disk space means potentially more fragmentation causing the R/W head to jump back and forth much more, not because it has to travel to the other end of the drive.
As to your question about 10%, that is just an arbitrary number. Many years ago when drives were much smaller, that was more or less a rule of thumb but today that no longer applies. Years ago, most users only had one drive and of course, it contained the OS, which needs free space to operate in.
You still need a nice chunk of free space on secondary drives to accommodate defragging tasks, and for temporary files. But 10% on a 8TB disk would be 800GB and reserving that amount is just ridiculously wasteful.
I think reserving anything over 100GB would be wasteful and on my own drives, I like to keep at least 30GB free. If I start crowding that, it is time to free up or buy more space.
But there are many variables that must be considered. What kind of data matters. If this disk is full of family photos, music or video files, they are likely to never be "
opened" for editing. That means they likely will never change size, need any temp file locations, or ever need to be moved. But if these files are document type files (Word files, spreadsheets, presentations, etc.) that will be opened, the OS always creates temporary copies when they are opened and that requires extra space. And if modified, the modified file is always saved in a new location then the old copy is deleted (creating a new hole of free space).