If this has to be changed only once at the beginning of a piece, the simplest is to say
\def\bottomfacteur{8}
before invoking
normalshapemediumseriesnormalshapemediumseries\debutmorceau. Note that
normalshapemediumseriesnormalshapemediumseries\bottomfacteur is invoked to compute the dimension register
normalshapemediumseriesnormalshapemediumseries\staffbotmarg whenever this register contains a zero dimension.
The number (it must be a number) is multiplied by
normalshapemediumseriesnormalshapemediumseries\Interligne and
affected to
normalshapemediumseriesnormalshapemediumseries\staffbotmarg.
If this has to be changed several times to meet page layout
requirements with scores sometimes – but only sometimes – going very deep
below the lowes staff of the systems, it is wiser to use
normalshapemediumseriesnormalshapemediumseries\advancebottom, namely:
\advancebottom{10pt}
to advance the bottom margin register by that dimension at
the next system line break.
REMARK : it is also possible to change the system top
margin, i.e.
normalshapemediumseriesnormalshapemediumseries\stafftopmarg between systems. Changing top and
bottom margin add together but it may influence the position of the whole in
the pages.