[stumped]

I am trying to extract individual stories from an omnibus edition
The way the collected stores book is assembled is that each individual book as it's own cover ( html page) followed buy it's text , and there is a master toc to select stories from

the extract process seems easy enough , I can delete all but the HTML that relates to a single story, delete unused images, save as... then use modify epub to clear out the unwanted TOC entries..

but I m stuck on 1 simple trick. the edited down "book" now has just two html files, one is the correct cover for that story, and one is the story text.
but I cannot figure out how to get calibre to treat that 1st html page as the new cover i.e to put that page ( text and image) into the calibre library as a cover picture. I tried saving a copy, editing with sigil outside of calibre - renaming that 1st file as cover.html, then adding that back to calibre but calibre still makes its own, different generic cover, or makes one with just the metadata words on it
what am I missing or doing wrong.