(that's actually how I solved the original issue that I reference in the post, and how I got the idea for this silly solution)

The filesystems where headroom matters are var, tmp and sometimes root. I like this strategy with logfiles because nethack.log.gz.30 was approximately as important as empty space. Keeping another 8gb on root and tmp seems extreme.

ncdu saves my bacon at least every few months. I do machine learning and am always running out of space!