How can I adjust length-fraction of tremolo on beamed stem?

[takumi ikeda]

\version "2.19.61"

{
  \override Stem.length = #10
  \override StemTremolo.beam-thickness = #0.3
  \override StemTremolo.slope = #0.2
  \override StemTremolo.beam-width = #1.5
  \override StemTremolo.extra-offset = #'(0 . 1)
  \override StemTremolo.length-fraction = #0.7     %% not works on the beams.
  %\override Beam.length-fraction = #0.7     %% would like to adjust
only tremolo, not beams.
  c''4:64 c''8.:64 c''16:64
}

% Thanks,
% Takumi