How do you handle Disk Space alerting?

In the past I've pretty much done it both ways. In general I try to stick with the built-in percent options, especially as MSPs with a wide range of environments. I've sometimes even used custom thresholds like 99.63768%. I've also written custom DataSources in the past. I don't recall the specifics, I think one of them may have even switched between using % and free space depending on the size of the drive (I don't have access to it anymore).