\documentclass[a4paper,twoside]{book}
\usepackage{gcheck}

\usepackage[margin=1cm,includehead]{geometry}
\geometry{reset,  a3paper, landscape}
\geometry{a6paper,  hmargin={1cm,0.5cm},  heightrounded}
\geometry{portrait,  a5paper, centering }
% `centering' (marginratio=1:1) is ineffective `hmargin'.
\geometry{dvips,  showframe}

\begin{document}

\typeout{}
\dcheck\paperwidth{423.94606pt}
\dcheck\paperheight{597.50787pt}
\dcheck\textwidth{381.26695pt}
\dcheck\textheight{418.0pt}
\dcheck\oddsidemargin{-43.81725pt}
\dcheck\evensidemargin{-58.04362pt}
\dcheck\topmargin{-12.71039pt}
\dcheck\headheight{12pt}
\dcheck\headsep{18.06749pt}
\dcheck\footskip{25.29494pt}
\dcheck\marginparwidth{115pt}
\dcheck\marginparsep{7.0pt}
\dcheck\columnsep{10pt}
\dcheck\hoffset{0pt}
\dcheck\voffset{0pt}
\ccheck\mag{1000}
\checkresults
\typeout{}

\manyfoxes\manyfoxes\manyfoxes\manyfoxes
\manyfoxes\manyfoxes\manyfoxes\manyfoxes
\end{document}