With your regular defrag habit I can't see any reason for concern. Performance may start to degrade at really low free space. For example, usually your Windows Paging file is set to Variable Size, meaning it can expand and shrink as needed. But it always has an upper limit of size, typically less than 10% of your hard drive, usually something only a little more than your RAM space - something like 2 to 4 GB. If you get below that the disk might start thrashing around trying to move data between RAM and Paging File. But by that time your Free Space will be under 10 GB, probably less, and you're nowhere near that situation.