What I want to do is easy, as I wrote: just copy the text and paste it into the Calculator app, and it does the same thing - including placing the result in the clipboard. Not a big deal, it’s just more convenient having it directly in the word processor.
One example of how I use this feature is during tax time. I have a Word document with a long table of expenses, which I update for the current year. That involves arithmetic on lists of numbers (either positive or negative), and then multiplying by a percentage to figure out, say, the percentage of our household expenses I can deduct for my home office/studio.
I’m sure there will be other features/commands I need to find as I move over to Libre Office. The reason I posted to ask about this one is that… I wanted to add something up! (But I’m not working on taxes now, thank goodness.)