computing staff line lenght in case of ragged-right

[Wilbert Berendsen]

Hi,

Still typesetting my hymn, I use ragged-right = ##t to make the lines not too 
long.

But I want the stafflines to be the full line-width. But how do I convert from 
staffspaces to milimeters?

Currently I use:

\version "2.11.43"
#(set-global-staff-size 18)

\paper {
  line-width = 10 \cm
}

\layout {
  ragged-right = ##t
  \context {
    \Staff
    \override StaffSymbol #'width = #'60
  }
}

But that's just a guess. I tried:
    \override StaffSymbol #'width = #(* 10 cm)

but that didn't work, the lines became very long... What would be the correct 
computation? TIA!
best regards,
Wilbert Berendsen

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