I took a look at those, links, thanks. Have to say I’m a little confused. Those linked docs are not really clear to me. I’m not sure of the series of operations I need to take to generally establish the base URL to use.
From what I can tell, this comes back to the title of my question about the URL to use when making a request. My approach so far had been to take the host component of the the user’s webid, and use that as the base URL when making requests. It sounds like that’s not general?