I know it is possible to get a rule to act as a filler either side of, 
say, a chapter head text by using \definefiller with alternative=rule 
and, as explained in an earlier post 
<https://tex.stackexchange.com/questions/497352/context-heading-text-surrounded-by-lines/497389#497389>, 
define a left and right version. So, for example, for the left version:

\definefiller
   [ChapterLeftRule]
   [ChapterRule]
   [left=\zerowidthnobreakspace,
    right=\quad]

This gives me a rule that extends to the margins. What if I want the 
rule to be, say, 2 cm either side of my centered header text? I tried 
experimenting by replacing left=\zerowidthnobreakspace with 
left=\quads[3] and that did shorten the rule somewhat, but when I did 
the same for the other side, it did not balance correctly. So maybe I 
need to deal with the 'left=' differently to achieve the desired 
result... or try an entirely different method!

Of course, I don't want the rule at the baseline, but raised, as the 
original stackexchange 
<https://tex.stackexchange.com/questions/497352/context-heading-text-surrounded-by-lines/497389#497389> 
example gives. Any clues?

Julian