With the following code:
(defvar-local my-text-width nil
"Text area width for current buffer, or nil if no attention needed.")
(put 'my-text-width 'safe-local-variable #'integerp)
(defun my--margin-setup ()
"Handle settings of `my-text-width'."
(walk-windows
(lambda (window)
(with-current-buffer (window-buffer window)
(let ((margin (and my-text-width
(/ (max 0 (- (window-total-width)
my-text-width))
2))))
(when (or (not (equal margin left-margin-width))
(not (equal margin right-margin-width)))
(setq left-margin-width margin)
(setq right-margin-width margin)
(set-window-buffer window (current-buffer))))))))
(add-hook 'window-configuration-change-hook #'my--margin-setup)
You can set my-text-width
in a file-local or directory-local variable, and Emacs will automatically set the margins accordingly to get that text width. It works even when you resize the frame or introduce splits.