According to my Grafana dashboard, the following metrics work nicely for alerting for available space,
100 - ((node_filesystem_avail_bytes{mountpoint="https://stackoverflow.com/",fstype!="rootfs"} * 100) / node_filesystem_size_bytes{mountpoint="https://stackoverflow.com/",fstype!="rootfs"})
The formula gives out the percentage of available space on the pointed disk. Make sure you include the mountpoint and fstype within the metrics.