幅と高さを適当に変えてみれ
中央に配置されるようになるはず

---

\textwidth=155mm % テキスト領域の幅
\textheight=230mm % テキスト領域の高さ

\newlength\twosideextramargin
\twosideextramargin3.5mm % twoside の場合のズレ幅

%%% centering the text area
\def\MarginCentering{
\topmargin=\paperheight
\advance\topmargin by -\textheight
\topmargin=.5\topmargin
\advance\topmargin by -1in
\oddsidemargin=\paperwidth
\advance\oddsidemargin by -\textwidth
\oddsidemargin=.5\oddsidemargin
\advance\oddsidemargin by -1in
\if@twoside
\evensidemargin=\oddsidemargin
\advance\oddsidemargin by \twosideextramargin
\advance\evensidemargin by -\twosideextramargin\fi}
\MarginCentering