I don't have custom columns for my "normal" library. The built-in columns work quite well for that.
For my fanfic library:
- status (text:tag) - in-progress or complete
- modified (datetime) - date last updated
- chapters (int)
- words (int)
- pair (text:tag) - pairings
- char (text:tag) - main characters
- tropes (text:tag)
- txtquery (comments) - placeholder for MultiColumnSearch TXT regex query results for metadata extraction
- tmptags (text:tag) - placeholder for tags (pair, char, tropes) prior to standardization/clean-up
- state (text:tag) - for tracking the status of ebook metadata processing and clean-up
I just use the built-in Tags column for tagging the fandom.
Honestly, I haven't used Calibre in years. I just download epub directly from Archive of Our Own and save to Dropbox (all done on the iPad/iPhone). Alas, that has resulted in ~4K fanfics inclusive of dupes which I'm now trying to beat into some semblance of order.