Seriously though, you do have a good question. Every environment is different. A stable environment with very little fluctuation can be a few hundred MB (plus whatever the OS needs for temporary files) away from capacity for years on end - set the alarm at that level plus 1. A drive that's used for archiving everything-ever-created in a video-editing shop will grow to infinity quite fast - set the alarm so you catch it in time to add more space and consider a second alarm that monitors for increases in the rate of growth. A "temp drive" that fluctuates wildly but has only hit 75% once and probably never will again can probably have the alarm set at 76%.