Retrieving Y-offset value in a music function how?

[Nick Payne]

I have a barre function that, as well as the horizontal line indicating 
the duration of the barre, also has a vertical line at the LH end 
indicating the lowest note that the barre covers. However, it's a bit 
unsatisfactory in that I have to initially enter a guess for the length 
of the vertical line in staff units, and then produce the output and see 
if my guess needs altering. If I could find the distance that the 
TextSpanner is above the centre of the staff, then I should be able to 
change that parameter to be the position on the staff of the end of the 
line, and calculate it's length from those two values. From the 
internals reference it looks like the value I need is in the Y-offset 
setting, but how do I retrieve this value into a variable?

\version "2.15.14"

% Parameters:
% fretnum: the fret number for the barre - eg "VII"
% osp: outside staff priority, in case this needs altering from the default
% dirn: #UP or #DOWN
% vdrop: height of vertical line at LH end
% shorten: pair indicating left & right padding of TextSpanner
% adjBreak: adjust LH end of TextSpanner line when it has wrapped to 
following stave
% adjEnd: adjustment of line end when it wraps to following stave
barre = #(define-music-function (parser location fretnum osp dirn vdrop 
shorten adjBreak adjEnd)
    (string? number? number? number? pair? number? number?)
#{
    \once \override TextSpanner #'outside-staff-priority = #$osp
    \once \override TextSpanner #'bound-details #'left #'text =
    \markup\small\bold { \concat {
        \postscript #(string-append
            "0.1 setlinewidth 0.7 -0.5 moveto 0 -"
            (number->string $vdrop)
            " rlineto 0.5 0 rlineto stroke")
        $fretnum " "
    } }
    \once \override TextSpanner #'style = #'line
    \once \override TextSpanner #'font-shape = #'upright
    \once \override TextSpanner #'direction = #$dirn
    %draw a bracket edge on RHS
    \once \override TextSpanner #'bound-details #'right #'text = 
\markup { \draw-line #(cons 0 (/ $dirn -1)) }
    % set alignment of line with reference to left text
    \once \override TextSpanner #'bound-details #'left 
#'stencil-align-dir-y = #CENTER
    % change X pos of LH and RH end as desired
    \once \override TextSpanner #'bound-details #'left #'padding = 
#(car $shorten)
    \once \override TextSpanner #'bound-details #'right #'padding = 
#(cdr $shorten)
    % allow adjustment of line end when it wraps to following stave
    \once \override TextSpanner #'bound-details #'right-broken 
#'padding = #$adjEnd
    % adjust LH end of line when it wraps to following stave so that it 
doesn't
    % extend to the left of the notes on the stave
    \once \override TextSpanner #'bound-details #'left-broken #'X = 
#$adjBreak
    % override to remove text and bracket edge at line breaks
    \once \override TextSpanner #'bound-details #'left-broken #'text = ##f
    \once \override TextSpanner #'bound-details #'right-broken #'text = ##f
#})

\relative c'' {
        \barre "III" #0 #UP #4 #'(-1.5 . -1) #10 #0.5 f4.\startTextSpan 
f8 b4. d8\stopTextSpan
}

barre.png
Description: PNG image