How to set line-width to width of paper

[Richard Shann]

I have just started using

\override #'(line-width . n)

for text markup, and I want the value n to be chosen so as to work out
as the distance between the margins of the paper I'm printing on. As it
is it is in staff spacing units which is inconvenient (I am writing the
critical commentary at the end of the work).

Could someone point me in the right direction here?

Richard Shann