I have downloaded the latest calibre file and installed it fresh. I have opened the user manual and gone thru adding the custom column and sub-column step-by-step and so forth. Long story short (too late!) - I can't get it to work.
I can create the custom column. And I can create a single sub-column under that. But that 2nd level will not allow any further refinement. Any attempt to create the 3rd layer results in a new 2nd layer sub-column that is is named x.y (where x is the first name and y is the sub-name).
I've tried searching in the forum and such but I cannot find an answer. Forgive me if this is a duplicate. I've included a doc that has the screenshots for each step in the attachments