[Groff] Horizontal rule w/ length a fraction of the page width?

[Michael Smith]

What is the recommended markup for rendering a horizontal line,
with the length of the horizonal line being some arbitrary
fraction of the global line length (that is, the distance from the
left margin to the right margin -- ll).

To specify a line whose length is 80 percent of the line length, I
am currently using this:

  \l'\n(.l/1.25'

The intent of that being to specify the value as "the line length
divided by 1.25" 

I don't know whether that is the correct way to specify "line
length divided by 1.25", but it seems to work, and seems to be
compatible with the *roff versions on the non-groff environments I
have tested on (older Solaris, HP-UX, and Dec/Tru64 -- all using
some version of AT&T troff, I guess.)

I am generating the groff markup programatically (via XSLT
conversion from DocBook XML sources). I want to eventually provide
a user-configurable parameter for controlling the length of the
line, and have the XSLT code for doing it be as clear and simple
as possible, for maintenance reasons (we have multiple developers
working on the stylesheets). It would not be too much trouble to
just have it set up so that users can specify the value for the
parameter in the way they would expect (as 80% or .80), and then
just have the XSLT code do the computation to divide 1 by that
number (if it's not zero), and generate \l'\n(.l/1.25'

But I'd actually rather just be able to have it generated "line
length times .80" or "80 percent of line length". I tried this:

  \l'\n(.l*.80'

But it is obviously wrong, because it causes grotty to emit a long
stream of error messages.

  --Mike

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