markup-command boxing the real printed ink?

[Thomas Morley]

Hi all,

consider:

\markup
  \override #'(box-padding . 0) \box \fontsize #15 \musicglyph #"f"

The bounding box of the forte-sign does not describe its real printed
ink and thus it sticks out of \box.
While this is on purpose and fine in almost all cases, I want to
define a markup-command boxing the real printed ink. The outcome
should be like the manual adjusted:

\markup
  \override #'(box-padding . 0)
  \box
  \with-dimensions #'(-2.3 . 9.92) #'(-3.85 . 10.8)
  \fontsize #15 \musicglyph #"f"

Hints?

Thanks,
  Harm