Defining correct font size and style of headings and footnotes

Question

I am trying to format my thesis' headings and footnotes according to my university's guidelines for doctoral theses. The guidelines has some clear specifications for for normal text, headings and footnotes:

• Normal textsize: 13pt
• Font: Calibri (all text)
• Heading 1: 18pt
• Heading 2: 16pt
• Heading 3: 14pt
• Footnotes: 12pt
• Indent: 4mm

Beyond the specified formatting guidelines, the formatting should follow normal guidelines from the relevant field of research, which in my case is APA-style.

By this description, headings should look something like below. (Also, the headings should be unnumbered.)

I found a partial solution here, but it does not well explain how i can define font sizes and indents. I've set indent length \setlength\parindent{4mm}. This works for paragraphs, though I'm not sure whether it applies to headings. Below I added a sample of the document settings, showing what may be relevant to this.

\documentclass[twoside, open=right, fontsize=13pt, DIV=calc, chapterprefix=true]{scrreprt}

\usepackage[a4paper,width=170mm,top=20mm,bottom=20mm,headsep=1em,footskip=2em,hcentering,bindingoffset=6mm]{geometry}
\usepackage{graphicx}
\usepackage{fontspec}
\usepackage{subcaption}% loads package caption
\usepackage[footsepline,headsepline,manualmark]{scrlayer-scrpage}% sets page style scrheadings automatically
\usepackage{scrhack}% added
\usepackage{setspace}
\usepackage{microtype}% added
\usepackage[hidelinks]{hyperref}

%Styles
\usepackage{lmodern}
\setmainfont{Calibri}
\setsansfont{Calibri}% added
\onehalfspacing 
%\renewcommand*\chaptermark[1]{\markboth{\Ifnumbered{chapter}{\chaptermarkformat}{}}{}}
\renewcommand{\chaptermark}[1]{\markboth{#1}{}}
\AfterTOCHead{\markboth{}{}}
\clearpairofpagestyles
\setlength\parindent{4mm}

Answer 1

Here is a suggestion for the uptodate KOMA-Script version 3.29:

\documentclass[twoside, open=right, fontsize=13pt, DIV=calc, chapterprefix=true]{scrreprt}
\usepackage{lipsum}% only for dummy text
\usepackage[a4paper,width=170mm,top=20mm,bottom=20mm,headsep=1em,footskip=2em,hcentering,bindingoffset=6mm]{geometry}
\usepackage{graphicx}
\usepackage{fontspec}
\usepackage{subcaption}
\usepackage[footsepline,headsepline,manualmark]{scrlayer-scrpage}% sets page style scrheadings automatically
\usepackage{scrhack}
\usepackage{setspace}
\usepackage{microtype}
\usepackage[hidelinks]{hyperref}

\usepackage{lmodern}
\setmainfont{Calibri}
\setsansfont{Calibri}
\onehalfspacing 

\AfterTOCHead{\markboth{}{}}
\clearpairofpagestyles

\setcounter{secnumdepth}{\numexpr\partnumdepth-1\relax}% all section levels should be unnumbered
\counterwithout{figure}{chapter}% figure number without chapter number
\counterwithout{table}{chapter}% table number without chapter number
\setparsizes{4mm}{0pt}{0pt plus 1fil}% change parindent

\renewcommand\raggedchapter{\centering}% center chapter titles

\RedeclareSectionCommand[
  font=\fontsize{18pt}{21.6pt}\selectfont,
  beforeskip=\dimexpr3.3\baselineskip+\parskip\relax,
  afterindent=true
]{chapter}
\RedeclareSectionCommand[
  font=\fontsize{16pt}{19.2pt}\selectfont,
  beforeskip=3.5ex plus 1ex minus .2ex,
  afterindent=true
]{section}
\RedeclareSectionCommand[
  font=\itshape\fontsize{14pt}{16.8pt}\selectfont,indent=\the\parindent,
  beforeskip=3.25ex plus 1ex minus .2ex,
  afterindent=true
]{subsection}
\RedeclareSectionCommand[
  font=\normalsize,
  beforeskip=1em plus .3ex minus .1ex,
  indent=\the\parindent,
  runin=true,
  afterskip=1ex
]{subsubsection}
\RedeclareSectionCommand[
  font=\mdseries\itshape,
  beforeskip=1ex plus .15ex minus .1ex,
  indent=\the\parindent,
  runin=true,
  afterskip=1ex
]{paragraph}

\makeatletter
\renewcommand\sectionlinesformat[4]{%
  \@hangfrom{\hskip#2#3}{#4\Ifstr{#1}{subsection}{.}{}}% dot after title for level=3 (subsection)
}
\makeatother
\renewcommand\sectioncatchphraseformat[4]{\hskip#2#3#4.}% dot after title for level≥4 (subsubsection, parargraph, ...)

\addtokomafont{footnote}{\fontsize{12pt}{14.4pt}\selectfont}

\begin{document}
\tableofcontents
\chapter{Level 1}
\lipsum[1-2]\footnote{Footnote text}
\section{Level 2}
\lipsum[3-4]
\subsection{Level 3}
\lipsum[5-6]
\subsubsection{Level 4}
\lipsum[7-8]
\paragraph{Level 5}
\lipsum[9-10]
\end{document}

Result: