Set custom page text/line width in memoir for sidepars

I wish to lay out a page using the memoir package so it has dimensions something like this:

 # <-- 21.6cm (Letter stock) --> # # Margin | Content | | Note | Margin # # 2.5cm | <- 10cm --> | | 5.4cm | 2.5cm # # | ^.6cm | # 

Unfortunately I seem to be overlooking something in my attempts. Here is an example of what I have tried:

\documentclass[oneside,12pt]{memoir} \usepackage{ragged2e} \usepackage{lipsum} \begin{document} \setlrmarginsandblock{2.53cm}{2.53cm}{*}% \settypeblocksize{20cm}{10cm}{*}% \sideparmargin{outer}% \renewcommand{\sideparfont}{\footnotesize}% \renewcommand{\sideparform}{\RaggedRight} \setlength\marginparwidth{5.4cm}% \setlength\marginparsep{0.6cm}% \setlength{\parindent}{0pt}% no par indent \checkandfixthelayout[nearest]% \sidepar[]{\lipsum[2]}\lipsum[1] Textwidth: \the\textwidth\newline Linewidth: \the\linewidth \end{document} 

The above ends up looking like this:

 # Margin | Content | | Note #| # 2.5cm | <-- 13.5cm --> | | 5.4cm #| 

The above when run will print a textwidth of 284.0pt, but linewidth of 383.0pt. In this case there are approximately 28.425pt/cm, so the textwidth is 10cm, but linewidth is 13.47cm. This would seem to be a clue to the issue at hand, as the text appears (based on visual inspection) to be suspiciously about 3.5cm too large.

Running the above, the memoir class shall output something like the following to the LaTeX log:

 Stock height and width: 794.96999pt by 614.295pt Top and edge trims: 0pt and 0pt Page height and width: 794.96999pt by 614.295pt Text height and width: 563pt by 284pt Spine and edge margins: 71.13188pt and 71.13188pt Upper and lower margins: 124.64378pt and 107.3262pt Headheight and headsep: 14.5pt and 19.8738pt Footskip: 30pt Columnsep and columnseprule: 10pt and 0pt Marginparsep and marginparwidth: 17.07182pt and 170.71652pt Sidecapsep and sidecapwidth: 7pt and 50pt Sidebarhsep and sidebarwidth: 7pt and 50pt Sidebarvsep and sidebartopsep: 14.5pt and 0pt Sidebarheight: 546.64995pt Sidefoothsep and sidefootwidth: 7pt and 50pt Sidefootvsep and sidefootheight: 14.5pt and 546.64995pt 

One would expect that running \settypeblocksize would change the size of the textblock, be it updating the textwidth, linewidth or otherwise. This is consistent with my understanding of the relationship of these variables - as explained here on tex.SE.

In any case, in this instance it would be quite helpful to understand

1. why the size of the textblock isn't being updated as expected;
2. why \linewidth differs from \textwidth in the amount it does; and
3. what one ought to be doing to set the appropriate width of a block of text to accommodate extra-large side-paragraphs as in the case here.