#(define-public (calc-x-offset grob)
  (ly:grob-property grob 'positioning-done)
  (let* ((shift (ly:grob-property grob 'toward-stem-shift 0.0))
         (note-head-location
          (ly:self-alignment-interface::centered-on-x-parent grob))
         (note-head-grob (ly:grob-parent grob X))
         (stem-grob (ly:grob-object note-head-grob 'stem)))

    (+ note-head-location
       ;; If the property 'toward-stem-shift is defined and the script
       ;; has the same direction as the stem, move the script accordingly.
       ;; Since scripts can also be over skips, we need to check whether
       ;; the grob has a stem at all.
       (if (ly:grob? stem-grob)
           (let ((dir1 (ly:grob-property grob 'direction))
                 (dir2 (ly:grob-property stem-grob 'direction))
                 (beams (ly:grob-property stem-grob 'beaming)))
             (if (and (equal? dir1 dir2)
                      (not (or (and (= dir2 UP)
                                    (pair? beams)
                                    (not (car beams)))
                               (and (= dir2 DOWN)
                                    (pair? beams)
                                    (not (cdr beams))))))
                 (let* ((common-refp (ly:grob-common-refpoint grob stem-grob X))
                        (stem-location
                         (ly:grob-relative-coordinate stem-grob common-refp X)))
                 
                   (* shift (- stem-location note-head-location)))
                 0.0))
           0.0))))

\version "2.13.18"

\relative c'' {
  \override Script #'X-offset = #calc-x-offset           
  \voiceOne
  c8-. d-. e-. f-.
  \voiceTwo  
  a,8-. b-. c-. d-.
}