Specify distance from page margin to header with geometry

Question

With geometry, the placement of the header is set by specifying the distance from the text body below to the header above with headsep:

\documentclass{article}
\usepackage[showframe, headsep = 5mm]{geometry}
\usepackage{lipsum, fancyhdr}
    \pagestyle{fancy}
    \fancyhf{}
    \fancyhead[R]{A header plus the page number: \thepage}
\begin{document}
\lipsum
\end{document}

In MS Word, on the other hand, the placement of the header is set by specifying the distance from the page margin above to the header below:

My problem now is that I am given a guide that tells me the settings in MS Word, and I have to figure out how to translate that into LaTeX. And this setting I can't figure out. How can I with the help of geometry set the distance from the page margin above to the header below (in which case I would just set headsep = 0pt)?

Answer 1

You should specify top together with the includehead setting:

\documentclass{article}

\usepackage[showframe,includehead,top=5mm]{geometry}

\usepackage{lipsum, fancyhdr}
\pagestyle{fancy}
\fancyhf{}
\fancyhead[R]{A header plus the page number: \thepage}
\begin{document}
\lipsum
\end{document}

From the geometry user's manual, Figure 2 displays the meaning of includehead (and includefoot):

If you now want to specify the distance from the top of the page (margin) to the top of the text body, additionally set headsep and headheight. For example, we can set headheight to \baselineskip by default, and then set

\geometry{
  showframe,
  includehead,
  top=5mm,
  headsep=\dimexpr20mm-\baselineskip,
  headheight=\baselineskip
}

to have the top of the body text exactly 25mm from the page top margin.

Answer 2

This I how I ended up doing it:

\documentclass{article}
\usepackage[showframe,
    tmargin = 24mm,% distance from page margin to body text
    headheight = 4.22mm,% height of header (the minimum required by fancyhdr)
    % tmargin (24mm) - headsep (X mm) - headheight (4.22mm) = 16mm (= the desired distance from page margin to header) =
    headsep = 3.78mm,% distance from body text to header    
    ]{geometry}
\usepackage{lipsum, fancyhdr}
    \pagestyle{fancy}
    \fancyhf{}
    \fancyhead[R]{A header plus the page number: \thepage}
\begin{document}
\lipsum
\end{document}

Answer 3

You could calculate the correct values manually since you’ll hav to set them only once, don’t you? If the values are changing quite often you can use this code

\makeatletter
\geometry{
   showframe,
   top=20mm,
   headsep = \dimexpr\Gm@tmargin-\headheight-5mm\relax,
}
\makeatother

Where \makeatletter enables @ as part of command names. Then in \dimexpr (dimension expression terminated by \relax) we us the top margin, which must be set before headsep width top (!), and subtract the \headheight and an additional 5mm. Resulting in the header starting 5mm from the top of the page.