As a rule of thumb, observe these rules:

Disregarding fragmentation issues, above 60% used on a harddrive noticeable performance degradation sets in. Above [60*sqrt2(=approx. 1.4)]=84% performance is virtually not worth mentioning, i.e. more than exponentially declines! Defragmenting might even not work.

20% of 100GB is 20Gb of free space for temporary files is too much..

I would typically maintain at least 10GB for application and OS swap space especially if the amt of physical memory is less than 1GB

my $0.02 worth of though..