It would be 'nice' if
Preferences->Add your own columns displayed a table with Column heading, Lookup name (with the '#' on custom columns), Column Type, Description, Format etc.
Then there'd be no excuse for missing the need for a prepended crunch
The output of the
calibredb custom_columns -d command is comprehensive, but it's not what I'd call 'user friendly'

Nor is it complete in that it only 'lists' custom columns.
BR