I want to create a column spanning 5 columns with m column specifier. It seems my calculation \multicolumn{5}{m{5\temp}|}{\hrulefill} is wrong as the rule does not occupy the left space. What is the correct calculation?
\documentclass[12pt]{article}
\usepackage[a6paper,landscape,left=3mm,right=3mm,top=3mm,bottom=12mm]{geometry}
\usepackage{longtable,array}
\tabcolsep=1mm\relax
\arrayrulewidth=1pt\relax
\renewcommand*{\arraystretch}{2.5}
\newlength\temp
\temp=\dimexpr\dimexpr\textwidth-7\arrayrulewidth-12\tabcolsep\relax/6\relax
\begin{document}
\begin{longtable}{|*{6}{m{\temp}|}}\hline
1 & 2 & 3 & 4 & 5 & 6 \\\hline
\LaTeX &\multicolumn{5}{m{5\temp}|}{\hrulefill}\\\hline
\end{longtable}
\end{document}

