The easiest way is to set the horizontal margin ratio to 1:1 and let memoir
recalculate and apply it:
\setlrmargins{*}{*}{1}
\checkandfixthelayout
The arguments to \setlrmargins
are spine margin, fore-edge margin, and ratio, if you omit a value by writing just*
it will be calculated.
If you don't like to use memoir
commands the specify page dimensions, you can check the original code how margins are set depending on the twoside
option. Originally it's:
\if@twoside
\setlength\@tempdima {\paperwidth}
\addtolength\@tempdima {-\textwidth}
\setlength\oddsidemargin {.4\@tempdima}
\addtolength\oddsidemargin {-1in}
\setlength\marginparwidth {.6\@tempdima}
\addtolength\marginparwidth{-\marginparsep}
\addtolength\marginparwidth{-0.4in}
\else
\setlength\@tempdima {\paperwidth}
\addtolength\@tempdima {-\textwidth}
\setlength\oddsidemargin {.5\@tempdima}
\addtolength\oddsidemargin {-1in}
\setlength\marginparwidth {.5\@tempdima}
\addtolength\marginparwidth{-\marginparsep}
\addtolength\marginparwidth{-0.8in} % don't know why this isn't .4
\fi
So, you could do the required part in your preamble:
\documentclass[openright,twoside]{memoir}
\usepackage[english]{babel}
\usepackage{blindtext}
\makeatletter
\setlength\@tempdima {\paperwidth}
\addtolength\@tempdima {-\textwidth}
\setlength\oddsidemargin {.5\@tempdima}
\addtolength\oddsidemargin {-1in}
\setlength\marginparwidth {.5\@tempdima}
\addtolength\marginparwidth{-\marginparsep}
\addtolength\marginparwidth{-0.8in} % don't know why this isn't .4
\setlength\evensidemargin\oddsidemargin
\makeatother
\begin{document}
\blindtext[15]
\chapter{test}
\end{document}
I additionally set \evensidemargin
equal to \oddsidemargin
. With this example you will see symmetric page layout, page 4 is empty and chapter 1 starts at page 5, which is odd.