relative tremolo vertical spacing

[Matthew]

Hi all

Is there a way to specify a relative vertical offset for a tremolo? I want to
(for example) make the tremolo appear on the stem halfway between the beam and
the notehead.

I can't use StemTremolo #Y-offset as I've got tremolos on crotchets ->
demisemiquavers, and I don't particularly want to redefine the offset each time.

eg:

{
\override Beam #'positions = #'(-5 . -5) 
d8:32( \times 2/3 {b16) d b} % this tremolo needs to be much higher
d16. b32:128( b16.) d32 % this tremolo can't be any higher
}