shortened Hairpin hack; how to get "width" of current ChordEvent?

[Kieren MacMillan]

Hello, all!

I'm trying to hack an easy way to end hairpins early... Here's what  
I've got so far (a variation on the dynamic+text function Nicolas  
helped me with):

%% CODE SNIPPET ENDS
\version "2.9.7"

dynNull =
#(define-music-function (parser location width event-chord)
                        (number? ly:music?)
   ;; add a null dynamic of desired width to the EventChord
   (set! (ly:music-property event-chord 'elements)
         (cons (make-music 'AbsoluteDynamicEvent
                           'text (markup #:hspace width))
                 (ly:music-property event-chord 'elements)))
   ;; return the chord with the right-aligned null dynamic attached
   (make-music 'SequentialMusic
     'elements
     (list (make-music 'ContextSpeccedMusic
             'property-operations '()
             'context-type 'Bottom
             'element (make-music 'OverrideProperty
                        'once #t
                        'grob-property-path (list 'self-alignment-X)
                        'grob-value 1
                        'symbol 'DynamicText
                        'pop-first #t))
           event-chord)))

\relative c'
{
        c\> c\! c\> \dynNull #2 c |
        c c c c
}
%% CODE SNIPPET ENDS

So, as you can see, you put in the "width" of the blank dynamic you  
want, and Lilypond attaches it (right-aligned) to the EventChord,  
thus forcing the hairpin to end "early". [OK, it ain't pretty -- but  
it's prettier than skipped notes in a separate Voice, or reduction  
factors which screw up the MIDI values, etc., for the sake of a  
visual adjustment.]

Although this works pretty well, the number isn't a percentage of the  
"natural width" of the EventChord, but a hard-coded value (in staff  
spaces) -- in other words, this value needs to be modified to adapt  
to different layouts/spacing.

Is there any way to make such a function "intelligent" enough that

        c\> \shortenHairpin #25 c

would shorten the Hairpin by 25% (of its "natural width") automatically?

Or is there a better way to do what I want, given the current state  
of the Dynamics code?

Thanks,
Kieren.