scheme in \layout to produce relative dimensions

[Graham Percival]

I'm trying to produce
line-width = line-width - 30mm

The docs contain
\paper {
  #(define bottom-margin (* 2 cm))
}
which I assume means  bottom-margin = bottom-margin*2cm.  If not, 
please let me know.  :)

Here's what I unsuccessfully tried,

\score{
  \relative c'' {
    \repeat unfold 16 {a4 b c d}
  }
  \layout{
%    left-margin = #(- 30 mm)
    #(define line-width (- 30 mm))
  }
}

Cheers,
- Graham