How can I define table column by width and align it using siunitx at the same time in a longtable?

Question

I have this MWE:

\documentclass[12pt,landscape,a4paper]{article}
\usepackage{multirow}
\usepackage{microtype}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{longtable}
\usepackage[utf8]{inputenc}
\usepackage[english,czech]{babel}
\usepackage{hyphenat}
\usepackage{siunitx,booktabs}
\sisetup{output-decimal-marker = {,}}
\usepackage{ragged2e}
\newcommand*\fixalign[2][]{\begin{tabular}{@{}S[#1]@{}}#2\end{tabular}}
\begin{document}
\begin{longtable}{p{4cm}ccS[table-format=1.1]}
\multirow{2}{4cm}{\strut \Centering Název} &
\multicolumn{2}{c}{\Centering Premiéra}  &
\multirow{2}{0.8cm}{\Centering Min. doba}  \\
 &\Centering Svět & \Centering ČR &  \\

\Centering Kazatel Kalašnikov &  1.2. & 3.4. & 5.8\\

\Centering Musíme si promluvit o Kevinovi &
\multirow{2}{*}{1.6.} &
\multirow{2}{*}{5.4} &
\multirow{2}{*}{\fixalign[table-format=1.1]{2.8}}\\
\end{longtable}
\end{document}

That compiles into this:

I would like 5.8 and 2.8 to be aligned. I figure I would need to define colums to have certain width and only then use siunitx, but I am not sure how to do that. (My actual table is much larger and slightly more complicated.)

Answer 1

As usual, multirow is not the answer; I removed the unnecessary packages, add them back if you really need them.

\documentclass[12pt,a4paper]{article}
% the following three should be first
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[english,czech]{babel}

\usepackage{makecell}
\usepackage{ragged2e}
\usepackage{longtable}
\usepackage{siunitx,booktabs}
\sisetup{output-decimal-marker = {,}}


\begin{document}

\begin{longtable}{
  >{\Centering}m{4cm}
  S[table-format=1.1]
  S[table-format=1.1]
  S[table-format=1.1]
}
Název & \multicolumn{2}{c}{Premiéra}  & {Min.} \\
      & {Svět} & {ČR}                 & {deba} \\

Kazatel Kalašnikov &  1.2 & 3.4 & 5.8\\

Musíme si promluvit o Kevinovi & 1.6 & 5.4 & 2.8 \\
\end{longtable}
\end{document}

Answer 2

A quick solution without \newcommands:

\documentclass[12pt,landscape,a4paper]{article}
\usepackage{multirow}
\usepackage{microtype}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{longtable}
\usepackage[utf8]{inputenc}
\usepackage[english,czech]{babel}
\usepackage{hyphenat}
\usepackage{siunitx,booktabs}
\sisetup{output-decimal-marker = {,}}
\usepackage{ragged2e}

\begin{document}
\begin{longtable}{p{4cm}ccS[table-format=1.1]}
\multirow{2}{4cm}{\strut \Centering Název} &\multicolumn{2}{c}{\Centering Premiéra}  & \multirow{2}{0.8cm}{\Centering Min. doba}  \\
 &\Centering Svět & \Centering ČR &  \\
\Centering Kazatel Kalašnikov &  1.2 & 3.4 & \multirow{1}{*}{5.8}\\
\Centering Musíme si promluvit o Kevinovi & \multirow{2}{*}{1.6} & \multirow{2}{*}{5.4} & \multirow{2}{*}{2.8}\\
\end{longtable}
\end{document}