Auto generate might have unwanted effects.
(I find it amazing how many self Pub's still use H tags for big and bolder. Chap #, Chap Title, Time-Place. That can make for a pretty busy TOC just for the Start of a chapter
)
Calibre already has a stand alone TOC tool (you can add it to your Toolbar) that allow a choice. (I think this is the exact same one as built into the book editor)