backspace makeupwidth tradeoff

["M.guravage" <m.guravage@pragma-cts.com>]

Hi,

I'd like to increase the backspace at the expense of the makeupwidth. I
like to do this by adding either a scalar or a percentage of the
backspace. In the following bit of code I'd expect both sets of
\mybackspace and \mywidth macros to yield identical results, but the
don't. The scalar approach seems to work correctly, and the percentage
not.

I'd appreciate it if somebody could show me where I've gone wrong. It must
be embarrassingly simple, but I can't see it - yet. Thanks in advance.

Cheers,

Michael


\showgrid
\showframe
%\showlayout

\def\abcor{1.25cm} % absolute binding correction
\def\pbcor{0.5}    % percentage binding correction

% default backspace is 2.5cm
%\def\mybackspace{\dimexpr(\backspace + \abcor)}
\def\mybackspace{\dimexpr(\backspace + \pbcor\backspace)}

% default makeupwidth is 15cm
%\def\mywidth{\dimexpr(\makeupwidth - \abcor)}
\def\mywidth{\dimexpr(\makeupwidth - \pbcor\backspace)}

\setuplayout
  [location=middle,
   backspace=\mybackspace,
   width=\mywidth]

\starttext
  \dorecurse{10}{\input tufte \par}
\stoptext