Layout: Fehlendes Maß bei headheight

\documentclass[landscape,ngerman]{scrartcl}
\setparsizes{0pt}{0pt}{0pt plus 1fil}
\def\Margin{2.75cm} % Testwert
\usepackage[showframe=true]{geometry}

\usepackage{lmodern}
\renewcommand*{\familydefault}{\sfdefault}

\usepackage{calc}

\usepackage{scrlayer-scrpage}
%\pagestyle{scrheadings} % Default
\setkomafont{pagehead}{\normalfont}
\ifoot{}
\cfoot{}% keine Seitenzahl oder Foot anzeigen!

\usepackage{tikz, mwe}
\begin{document}
\def\Head{Head \\ head \\ head \\ head \\ head}

\newlength\HeadHeight
\settototalheight\HeadHeight{\vbox{\Head}} % Kopfhöhe messen

\newgeometry{
headheight=\HeadHeight+0mm, % Kopfhöhe einsetzen
margin=\Margin,
includehead,   
headsep=0mm,%
}
HeadHeight: \the\HeadHeight  \\
\ihead{\Head}
%\chead{\Head}

\begin{tikzpicture}[
shift={(current page.south west)}, overlay, remember picture,
green!50!black, <->, ultra thick,]
\draw[red] (current page.north) -- ([yshift=-\Margin]current page.north) node[midway, right]{margin SCHLECHT};

\draw[] (current page.south) -- ([yshift=\Margin]current page.south) node[midway, right]{margin};
\draw[] (current page.west) -- ([xshift=\Margin]current page.west) node[midway, above]{margin};
\draw[] (current page.east) -- ([xshift=-\Margin]current page.east) node[midway, above]{margin};
\draw[gray] ([yshift=-4cm, xshift=\Margin]current page.west) -- ([yshift=-4cm, xshift=\textwidth+\Margin]current page.west) node[midway, above]{textwidth};
\draw[gray] ([xshift=-4cm, yshift=\Margin]current page.south) -- ([xshift=-4cm, yshift=\textheight+\Margin]current page.south) node[midway, right]{textheight};
\end{tikzpicture}

\end{document}