\documentclass[10pt,a4paper,twoside]{article}

\usepackage{gcheck}
\usepackage{calc}
\usepackage[showframe]{geometry}

\geometry{textwidth=\widthof{\texttt{M}}*80,
          lines=40+5*2-4, centering, includeheadfoot,
          top=\heightof{\texttt{N}}*10}

\setlength{\parindent}{0pt}
\begin{document}
{\ttfamily
MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM%
MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM%
% 80 characters
}

\bigskip
\manyfoxes\manyfoxes\manyfoxes\manyfoxes
\manyfoxes\manyfoxes\manyfoxes\manyfoxes

\typeout{}
\dcheck\paperwidth{597.50787pt}
\dcheck\paperheight{845.04684pt}
\dcheck\textwidth{419.99634pt}
\dcheck\textheight{550.0pt}
\dcheck\oddsidemargin{16.48578pt}
\dcheck\evensidemargin{16.48578pt}
\dcheck\topmargin{-11.15884pt}
\dcheck\headheight{12pt}
\dcheck\headsep{25pt}
\dcheck\footskip{30pt}
\dcheck\marginparwidth{111.0pt}
\dcheck\marginparsep{11pt}
\dcheck\columnsep{10pt}
\dcheck\hoffset{0pt}
\dcheck\voffset{0pt}
\ccheck\mag{1000}
\checkresults
\typeout{}

\end{document}