What I a good amount of disk space to have free?

Is there a general rule of thumb? IE 70, 80 or 90%?

It would be entirely dependent on how you used the storage and how large it is.

For example it used to be quite common to reserve 10% of disk space for "overhead" in some operating systems. This wasn't a lot when disks sizes were less than 1 GB but once large storage arrays became popular it could end up being quite a bit.

If you have a fairly static installation where you're trimming log files and not adding to a database then having very little free makes sense as otherwise you might be wasting the space. However, if you have a database or log file that grows by several GB per minute your initial free space should be very large. I used to work at a job where we added new versions of firmware on a rapid basis so we had actually automated to a certain extent the creation of new logical volumes and filesystems on same when the existing storage hit around 80% capacity. In my current job we have an application that is so sensitive to running out of space that we alert on it when it hits 50% whereas for most others we don't alert until 90%.

Thanks!