@[email protected] Unfortunately I don't know the difference as a fixed number; the level inbetween has a max-width set (and desired by the user) so the difference between its width and the "ancestor" depends on current viewport width. I was hoping there was some cunning way to infer it in "modern" css (I poke UIs occasionally, but the last time I did it full time was over twenty years ago).
Turns out searching for "make thing the same width as other thing in CSS" is surprisingly hard to specify tightly enough to get meaningful search results. Who would have thought...