Thanks, everyone, for the encouragement and helpful suggestions.

I think I just didn't copy the room carefully enough the first time, though.

So this time, I decided to place guides!

I have a vertical guide marking the difficult corner, and I have horizontal guides marking the two visible floor corners, and a couple of other guides to mark edges of things that it's hard to see the edges of when I'm zoomed in. I turned on the snap to guide option, and so far it's working for me, though I still have to delete an occasional layer in this new room whenever I forgot to move to the right layer to do something on that layer.

But when I have enough finished to show it, I'll show everyone what it looks like, and if you like it I'll happily share it.