set Hairpin length as percentage of "normal"?

[Kieren Richard MacMillan]

[ Lilypond 2.9.4-3 ]

Hey, y'all!

Just wondering if there's a way to set the length of a Hairpin as a  
percentage of its "natural" length (as determined by Lilypond) -- for  
example, something like

    \once \override Voice.Hairpin #'length = #0.8

I'm trying to come up with the easiest way to make Hairpins end on a  
non-note event (e.g., rest, barline, mid-measure).

Thanks,
Kieren.