insert-string-centered / column before $ appears?

[Emanuel Berg via Users list for the GNU Emacs text editor <help-gnu-emacs@gnu.org>]

(defun insert-string-centered (string &optional far-right)
  (interactive
   `(,(read-from-minibuffer "string: ")
     ,(string-to-number
       (read-from-minibuffer "far-right column [default 69]: ") )))
  (let*((max         (if (< 0 far-right) far-right 69))
        (str-len     (length string))
        (padding     (+ (/ (- max str-len) 2)
                        (if (= 0 (mod str-len 2)) 1 0)))
        (padding-str (make-string padding ?\ )))
    (insert padding-str string) ))

One (?) issue: 69 which is hardcoded above is the column that
appears right before the $ char appears in Emacs.

I take it this is stored somewhere?

Tested on this file: https://dataswamp.org/~incal/FILM

Source: https://dataswamp.org/~incal/emacs-init/edit.el

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