Dashed Stem Fontsize !

[Ya Gloops]

Hello ! I try to keep the proportion of the "dashed-stem" but I can't... If 
someone can help me.
Gilles

\version "2.23.10"


%#(set-global-staff-size 30)


#(define (make-round-filled-box x1 x2 y1 y2 blot-diameter)
   (ly:make-stencil (list 'round-filled-box (- x1) x2 (- y1) y2 blot-diameter)
     (cons x1 x2)
     (cons y1 y2)))


#(define (build-pos-list len on off)
   (let ((lst '(0)))
    
     (define (helper)
       (let ((bottom (+ (car lst) on)))
         (if (< bottom len)
             (begin
              (set! lst (cons bottom lst))
              (let ((top (+ (car lst) off)))
                (if (< top len)
                    (begin
                     (set! lst (cons top lst))
                     (helper))
                    (set! lst (cons len lst)))))
             (set! lst (cons len lst)))))
     (helper)       
     (reverse lst)))


#(define (dashed-stem on off)
   (lambda (grob)
     (let* ((blot (ly:output-def-lookup (ly:grob-layout grob) 'blot-diameter))
            (stencil (ly:stem::print grob))
            (X-ext (ly:stencil-extent stencil X))
            (thickness (interval-length X-ext))
            (Y-ext (ly:stencil-extent stencil Y))
            (len (interval-length Y-ext))
            (new-stencil empty-stencil)
            (factors (build-pos-list len on off)))
       
       (define (helper args)
         (if (<= 2 (length args))
             (begin
              (set! new-stencil
                    (ly:stencil-add
                     new-stencil
                     (ly:stencil-translate-axis
                      (make-round-filled-box (/ thickness -2) (/ thickness 2)
                        (car args) (cadr args)
                        blot)
                      (interval-start Y-ext)
                      Y)))
              (helper (cddr args)))
             new-stencil))


       (if (or (zero? on) (zero? off))
           stencil
           (helper factors)))))


dashedStems =
#(define-music-function (on off) (number? number?)
   #{
     \override Stem.stencil = #(dashed-stem on off)
   #})




\relative c'' {
  \dashedStems #0.5 #0.4
  c4
}


\score
{
  \relative c'' { \magnifyStaff #(magstep 10)
                  \dashedStems #0.5 #0.4
                  c4
  }
}