scrpage2 conflict with fancyhdr

Question

This is a followup to How can I add “page # of ##” on my document?

The MWE below works fine with the scrpage2 line commented out, but not with. With that line uncommented, I get the error:

Class scrlttr2 Warning: Usage of package `fancyhdr' together
(scrlttr2)              with a KOMA-Script class is not recommended.
(scrlttr2)              I'd suggest to use 
(scrlttr2)              package `scrpage2' or `scrlayer-scrpage'.
(scrlttr2)              Nevertheless, using requested
(scrlttr2)              package `fancyhdr' on input line 7.

(/usr/share/texlive/texmf-dist/tex/latex/fancyhdr/fancyhdr.sty

! LaTeX Error: Command \chead already defined.
               Or name \end... illegal, see p.192 of the manual.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              

l.270 ...chead}{\@ifnextchar[{\@xchead}{\@ychead}}

So the scrlttr2 recommendation is apparently to move from fancyhdr to something based on scrpage2/scrheadings, which I'm already using for \rohead. How would do this? If I get an answer, I'll update my answer to the “page # of ##” question.

\documentclass[12pt]{scrlttr2}    
%\usepackage{scrpage2}
%\pagestyle{scrheadings}
%\rohead{\jobname.tex\hspace{0.5cm}\today}

\usepackage{lastpage}
\usepackage{fancyhdr}
\pagestyle{fancy}
\renewcommand{\headrulewidth}{0pt}
\fancyfoot[C]{\thepage\ of \pageref{LastPage}}
\fancypagestyle{plain}{%
  \renewcommand{\headrulewidth}{0pt}%
  \fancyhf{}%
  \fancyfoot[C]{\thepage\ of \pageref{LastPage}}}

\usepackage[T1]{fontenc}
\usepackage{fouriernc}

\begin{document}

\def\today{10th December, 2015}
\begin{letter}{To Address}

  \opening{Addressee}

  \closing{Yours Sincerely,}

\end{letter}
\end{document}

Answer 1

With KOMA-Script version 3.19 or newer and documentclass scrlttr2 there is no need to load an additional package to get page # of ## in your letter(s). You only have to redefine \pagemark using \pageref{\thisletter.lastpage}:

\renewcommand\pagemark{{\usekomafont{pagenumber}%
  \pagename\nobreakspace\thepage\nobreakspace
    of\nobreakspace\pageref{\thisletter.lastpage}%
}}

If the page number should also be on the first page add the following line:

\setkomavar{firstfoot}{\parbox{\linewidth}{\centering\pagemark}}

MWE:

\documentclass[12pt]{scrlttr2}[2015/10/03]

\setkomavar{firstfoot}{\parbox{\linewidth}{\centering\pagemark}}
\renewcommand\pagemark{{\usekomafont{pagenumber}%
  \pagename\nobreakspace\thepage\nobreakspace
    of\nobreakspace\pageref{\thisletter.lastpage}%
}}

\usepackage[T1]{fontenc}
\usepackage{fouriernc}
\usepackage{blindtext}
% only for dummytext
\begin{document}
\setkomavar{date}{10th December, 2015}% note that I have changed the koma variable date instead \today

\begin{letter}{To Address}
  \opening{Addressee}
  \Blindtext[10]
  \closing{Yours Sincerely,}
\end{letter}
\end{document}

Or with your headline:

\documentclass[12pt]{scrlttr2}[2015/10/03]
\usepackage{scrlayer-scrpage}
\ohead*{\jobname.tex\hspace{0.5cm}\usekomavar{date}}

\setkomavar{firstfoot}{\parbox{\linewidth}{\centering\pagemark}}
\renewcommand\pagemark{{\usekomafont{pagenumber}%
  \pagename\nobreakspace\thepage\nobreakspace
    of\nobreakspace\pageref{\thisletter.lastpage}%
}}

\usepackage[T1]{fontenc}
\usepackage{fouriernc}
\usepackage{blindtext}
% only for dummytext
\begin{document}

\setkomavar{date}{10th December, 2015}
\begin{letter}{To Address}
  \opening{Addressee}
    \Blindtext[10]
  \closing{Yours Sincerely,}
\end{letter}
\end{document}

It is also possible to print a second letter with correct pagenumbers in the same document:

\documentclass[12pt]{scrlttr2}[2015/10/03]
\usepackage{scrlayer-scrpage}
\ohead*{\jobname.tex\hspace{0.5cm}\usekomavar{date}}

\setkomavar{firstfoot}{\parbox{\linewidth}{\centering\pagemark}}
\renewcommand\pagemark{{\usekomafont{pagenumber}%
  \pagename\nobreakspace\thepage\nobreakspace
    of\nobreakspace\pageref{\thisletter.lastpage}%
}}

\usepackage[T1]{fontenc}
\usepackage{fouriernc}
\usepackage{blindtext}% only for dummytext
\begin{document}
\setkomavar{date}{10th December, 2015}

\begin{letter}{To Address}
  \opening{Addressee}
    \Blindtext[10]
  \closing{Yours Sincerely,}
\end{letter}

\begin{letter}{To Address}
\setkomavar{subject}{Second letter}
  \opening{Second Addressee}
    \Blindtext[20]
  \closing{Yours Sincerely,}
\end{letter}
\end{document}

---

Info for older versions: With KOMA-Script version 3.18 or older you have to load package lastpage.

\documentclass[12pt]{scrlttr2}
\usepackage{lastpage}
%\usepackage{scrlayer-scrpage}
%\ohead*{\jobname.tex\hspace{0.5cm}\usekomavar{date}}
\setkomavar{firstfoot}{\parbox{\linewidth}{\centering\pagemark}}
\renewcommand\pagemark{{\usekomafont{pagenumber}%
  \pagename\nobreakspace\thepage\nobreakspace
    of\nobreakspace\pageref{\thisletter.lastpage}%
}}

\usepackage[T1]{fontenc}
\usepackage{fouriernc}
\usepackage{blindtext}
\begin{document}

\setkomavar{date}{10th December, 2015}
\begin{letter}{To Address}

\opening{Addressee}
\Blindtext[10]
\closing{Yours Sincerely,}

\end{letter}
\end{document}

---

Additionally here is an example for other classes with only one pagenumbering scheme. If you load both packages scrlayer-scrpage and lastpage you can redefine \pagemark:

\documentclass{book}
\usepackage{lastpage}
\usepackage{scrlayer-scrpage}
\renewcommand\pagemark{{\usekomafont{pagenumber}%
  \pagename\nobreakspace\thepage\nobreakspace
    of\nobreakspace\pageref{\thisletter.lastpage}%
}}

\usepackage{blindtext}
\begin{document}
\blinddocument
\end{document}