Indent Section Heading for KOMA Class

Question

Is there a way to make the indent of chapter and section so that the title of both starts math the same width? For the example: E of Einleitung and Z of Zielstellung directly aligned.

I am using this KOMA-class:

\documentclass[12pt, numbers=noenddot,parskip,headings=small,headsepline,listof=nochaptergap, listof=totoc,index=totoc]{scrreprt}
\usepackage[a4paper, includehead, left=3.5cm, right=1.7cm, top=2.5cm, bottom=2.0cm]{geometry}
\RedeclareSectionCommand[
beforeskip=-1sp,
afterskip=.25\baselineskip]{chapter}
\RedeclareSectionCommand[
beforeskip=-\baselineskip,
afterskip=.5\baselineskip]{section}
\RedeclareSectionCommand[
beforeskip=-.75\baselineskip,
afterskip=.5\baselineskip]{subsection}
\RedeclareSectionCommand[
beforeskip=-.5\baselineskip,
afterskip=.25\baselineskip]{subsubsection}
\RedeclareSectionCommand[
beforeskip=.5\baselineskip,
afterskip=-1em]{paragraph}
\RedeclareSectionCommand[
beforeskip=-.5\baselineskip,
afterskip=-1em]{subparagraph}

\begin{document}
\chapter{Einleitung}
\section{Zielstellung}
\end{document}

Answer 1

You can put the numbers in a box of the same width. Note that you cannot use em or ex here, because this would depend on the font sizes of the headings and therefore only be the same if the font sizes of all sectioning levels are the same, e.g.,

\newcommand*{\chapterformat}{%
  \IfChapterUsePrefixLine{\chapapp\nobreakspace\thechapter\autodot}{%
    \makebox[40pt][l]{\thechapter\autodot\hfil}%
  }%
}
\newcommand*{\sectionformat}{\makebox[40pt][l]{\thesection\autodot\hfil}}

This can be extended for all levels, but in this case 40pt are not enough:

\renewcommand*{\chapterformat}{%
  \IfUsePrefixLine{\chapapp\nobreakspace\thechapter\autodot}{%
    \makebox[80pt][l]{\thechapter\autodot\hfil}%
  }%
}
\renewcommand*{\sectionformat}{\makebox[80pt][l]{\thesection\autodot\hfil}}
\renewcommand*{\subsectionformat}{\makebox[80pt][l]{\thesubsection\autodot\hfil}}
\renewcommand*{\subsubsectionformat}{\makebox[80pt][l]{\thesubsubsection\autodot\hfil}}
\renewcommand*{\paragraphformat}{\makebox[80pt][l]{\theparagraph\autodot\hfil}}
\renewcommand*{\subparagraphformat}{\makebox[80pt][l]{\thesubparagraph\autodot\hfil}}

But if (and only if) you want the same indent for all levels from chapter down to subparagraph you could also redefine \chapterlinesformat, \sectionlinesformat and \sectioncatchphraseformat:

\documentclass[12pt, numbers=noenddot,parskip,headings=small,headsepline,listof=nochaptergap, listof=totoc,index=totoc]{scrreprt}
\usepackage[a4paper, includehead, left=3.5cm, right=1.7cm, top=2.5cm, bottom=2.0cm]{geometry}
\RedeclareSectionCommand[
beforeskip=-1sp,
afterskip=.25\baselineskip]{chapter}
\RedeclareSectionCommand[
beforeskip=-\baselineskip,
afterskip=.5\baselineskip]{section}
\RedeclareSectionCommand[
beforeskip=-.75\baselineskip,
afterskip=.5\baselineskip]{subsection}
\RedeclareSectionCommand[
beforeskip=-.5\baselineskip,
afterskip=.25\baselineskip]{subsubsection}
\RedeclareSectionCommand[
beforeskip=.5\baselineskip,
afterskip=-1em]{paragraph}
\RedeclareSectionCommand[
beforeskip=-.5\baselineskip,
afterskip=-1em]{subparagraph}

\newlength{\allsectionnumberwidth}
\setlength{\allsectionnumberwidth}{80pt}
\makeatletter
\renewcommand{\chapterlinesformat}[3]{%
  \IfArgIsEmpty{#2}{#3}{%
    \@hangfrom{\makebox[\allsectionnumberwidth][l]{#2}}{#3}%
  }%
}
\renewcommand{\sectionlinesformat}[4]{%
  \IfArgIsEmpty{#3}{\@hangfrom{\hskip #2}{#4}}{%
    \@hangfrom{\makebox[\allsectionnumberwidth][l]{\hskip #2#3}}{#4}%
  }%
}
\renewcommand{\sectioncatchphraseformat}[4]{%
  \IfArgIsEmpty{#3}{\hskip #2#3#4}{%
    \makebox[\allsectionnumberwidth][l]{\hskip #2#3}#4%
  }%
}
\makeatother

\setcounter{secnumdepth}{\subparagraphnumdepth}% To show it for all levels

\begin{document}
\chapter{Chapter Heading}
\section{Section Heading}
\subsection{Subsection Heading}
\subsubsection{Subsubsection Heading}
\paragraph{Paragraph Heading}
\subparagraph{Subparagraph Heading}
\end{document}

You even could detect the needed number width automatically:

\documentclass[12pt, numbers=noenddot,parskip,headings=small,headsepline,listof=nochaptergap, listof=totoc,index=totoc]{scrreprt}
\usepackage[a4paper, includehead, left=3.5cm, right=1.7cm, top=2.5cm, bottom=2.0cm]{geometry}
\RedeclareSectionCommand[
beforeskip=-1sp,
afterskip=.25\baselineskip]{chapter}
\RedeclareSectionCommand[
beforeskip=-\baselineskip,
afterskip=.5\baselineskip]{section}
\RedeclareSectionCommand[
beforeskip=-.75\baselineskip,
afterskip=.5\baselineskip]{subsection}
\RedeclareSectionCommand[
beforeskip=-.5\baselineskip,
afterskip=.25\baselineskip]{subsubsection}
\RedeclareSectionCommand[
beforeskip=.5\baselineskip,
afterskip=-1em]{paragraph}
\RedeclareSectionCommand[
beforeskip=-.5\baselineskip,
afterskip=-1em]{subparagraph}

\newlength{\allsectionnumberwidth}
\setlength{\allsectionnumberwidth}{40pt}% use at least 40pt
\newlength{\maxallsectionnumberwidth}
\setlength{\maxallsectionnumberwidth}{\allsectionnumberwidth}
\makeatletter
\renewcommand{\chapterlinesformat}[3]{%
  \IfArgIsEmpty{#2}{#3}{%
    \updateallsectionnumberwidth{#2}%
    \@hangfrom{\makebox[\allsectionnumberwidth][l]{#2}}{#3}%
  }%
}
\renewcommand{\sectionlinesformat}[4]{%
  \IfArgIsEmpty{#3}{\@hangfrom{\hskip #2}{#4}}{%
    \updateallsectionnumberwidth{\hskip #2#3}%
    \@hangfrom{\makebox[\allsectionnumberwidth][l]{\hskip #2#3}}{#4}%
  }%
}
\renewcommand{\sectioncatchphraseformat}[4]{%
  \IfArgIsEmpty{#3}{\hskip #2#3#4}{%
    \updateallsectionnumberwidth{\hskip #2#3}%
    \makebox[\allsectionnumberwidth][l]{\hskip #2#3}#4%
  }%
}
\newcommand*{\updateallsectionnumberwidth}[1]{%
  \begingroup
    \settowidth{\@tempdima}{#1}%
    \ifdim\@tempdima>\maxallsectionnumberwidth
      \global\maxallsectionnumberwidth\@tempdima
    \fi
  \endgroup  
}
\BeforeClosingMainAux{%
  \if@filesw
    \ifdim\maxallsectionnumberwidth>\allsectionnumberwidth
      \typeout{Rerun to get section number width right!}%
      \immediate\write\@auxout{\unexpanded{%
          \providecommand*\setallsectionnumberwidth[1]{}^^J%
        }%
        \string\setallsectionnumberwidth{\the\maxallsectionnumberwidth}%
      }%
    \fi
  \fi
}
\newcommand*{\setallsectionnumberwidth}[1]{%
  \setlength{\allsectionnumberwidth}{#1}%
  \global\allsectionnumberwidth\allsectionnumberwidth
}
\makeatother

\setcounter{secnumdepth}{\subparagraphnumdepth}% To show it for all levels

\begin{document}
\chapter{Chapter Heading}
\section{Section Heading}
\subsection{Subsection Heading}
\subsubsection{Subsubsection Heading}
\paragraph{Paragraph Heading}
\subparagraph{Subparagraph Heading}
% Compare after adding
%\setcounter{section}{99}\setcounter{subparagraph}{999}\subparagraph{More}
\end{document}

Note: You need at least two LaTeX runs to get the number widths correct, e.g., after activating the commented line before the \end{document}.

See https://tex.stackexchange.com/a/357254/9057 if you want something like this on a per page basis instead of a per document basis.

Answer 2

You could use something like

\newdimen\sectionnumberwdmax
\setlength\sectionnumberwdmax{1.25cm}% <- space needed for the section numbers

\renewcommand*{\chapterformat}{%
  \mbox{\chapappifchapterprefix{\nobreakspace}%
    \IfUsePrefixLine
      {\thechapter\autodot}
      {\makebox[\sectionnumberwdmax][l]{\thechapter\autodot\hfil}}%
  }%
}
\renewcommand*\sectionformat{\makebox[\sectionnumberwdmax][l]{\thesection\autodot\hfil}}

Example:

Code:

\documentclass[12pt, numbers=noenddot,parskip,headings=small,headsepline,listof=nochaptergap, listof=totoc,index=totoc]{scrreprt}
\usepackage[a4paper, includehead, left=3.5cm, right=1.7cm, top=2.5cm, bottom=2.0cm]{geometry}
\RedeclareSectionCommand[
  beforeskip=-1sp,
  afterskip=.25\baselineskip
]{chapter}
\RedeclareSectionCommand[
  beforeskip=-\baselineskip,
  afterskip=.5\baselineskip
]{section}
\RedeclareSectionCommand[
  beforeskip=-.75\baselineskip,
  afterskip=.5\baselineskip
]{subsection}
\RedeclareSectionCommand[
  beforeskip=-.5\baselineskip,
  afterskip=.25\baselineskip
]{subsubsection}
\RedeclareSectionCommands[
  beforeskip=.5\baselineskip,
  afterskip=-1em
]{paragraph,subparagraph}

\newdimen\sectionnumberwdmax
\setlength\sectionnumberwdmax{1.25cm}% <- space needed for the section numbers

\renewcommand*{\chapterformat}{%
  \mbox{\chapappifchapterprefix{\nobreakspace}%
    \IfUsePrefixLine
      {\thechapter\autodot}
      {\makebox[\sectionnumberwdmax][l]{\thechapter\autodot\hfil}}%
  }%
}
\renewcommand*\sectionformat{\makebox[\sectionnumberwdmax][l]{\thesection\autodot\hfil}}

\begin{document}
\chapter{Einleitung}
\section{Zielstellung}
\setcounter{section}{9}
\section{Abschnitt mit zweistelliger Nummer}
\end{document}