stem length and position in cluster

[Stefan Thomas]

Dear lilypond-users,
in the below quoted emxample, in my ponion, the stem from the b flat to the d should end at a little higher position. But I don't know, how to change it.
Here the example:

 schiefrunter = {  \once\override Stem #'extra-offset = #'(-1.1 . 0.75) \once\override Stem #'rotation = #'(-45 0 0) \once \override Stem #'length = #9 }
 \relative c'' { \context Voice << { <d a' b c>16} {s64 \schiefrunter bes'4*3/16}>> r16 r8 r4 r2}