Changing the length of voice follower lines

[DJF]

I’ve got a piano piece with many voice follower lines from one hand to the 
other. Most often the lines end up being too long and unsightly, extending 
through beams and stems. I know how to move them with 
VoiceFollower.extra-offset, but I can’t figure out how to change their length. 
I messed around with other things, like extents, but I don’t really understand 
those (I understand them theoretically, but not practically) and, anyway, 
nothing ever gets changed regardless of the values I use. Can’t help but think 
there’s probably an obvious answer, but I’m certainly not finding it!

What could I add below to shorten the right (top) end of the voice-follow line? 

Thanks for any help.

--
Dan


\version "2.19.58"

\score {
     \new PianoStaff 
       <<
     \new Staff = "right" \relative c'
     { \clef treble <g ees'>2. f'4 }

    \new Staff = "left" \relative c
    { \showStaffSwitch
      \clef bass r4 bes ees4 
      \change Staff = "right" \stemDown d'8 f, }
      >>
}