[NTG-context] calculations

[Henning Hraban Ramm]

Sorry, I skimmed the TeXbook for an hour but couldn't find how to do  
simple calculations in TeX...

This is a snippet from t-lilypond.tex:

\def\LPscale{0.6315789} % downscale the image
    \ifvmode
       \getfiguredimensions[\bufferprefix lilypond-\the\lily! 
figures.pdf][]%
%      \def\FigWidth{\LPscale\figurewidth}
       \leavevmode
       \ifdim\FigWidth>\localhsize
         \!!dimena=\localhsize
         \advance\!!dimena by-\FigWidth
         \noindent\hskip\!!dimena
       \fi
    \fi
    \externalfigure[\lily!img][scale=1000\LPscale]%
    \egroup%

I need to downscale every score to 63%, and the shown width check  
should use the corrected width.
How can I do that? (FigWidth should be LPscale * figurewidth)


Greetlings from Lake Constance!
Hraban
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