thumbs: Längenmaß mit @-Symbol vom Dokument aus ändern

\documentclass[ngerman, fontsize=9pt, paper=a5, 
twoside, automark,numbers=enddot,
headsepline,plainheadsepline,
]{scrreprt}

\usepackage{selinput}
\SelectInputMappings{adieresis={ä},germandbls={ß}}
\usepackage[ngerman]{babel}
\usepackage[T1]{fontenc}

% Layout
\usepackage[showframe,
left=15mm, right=15mm, % IST
%left=9mm, right=9mm % SOLL
marginparsep=0.5em, 
]{geometry}

\usepackage{marginnote}

% Fonts 
\setkomafont{pageheadfoot}{\normalfont\footnotesize}
\newcommand\parnumstyle[1]{\textsf{\bfseries\footnotesize#1}}

\usepackage[automark]{scrlayer-scrpage}[]
\ohead*{\marginnote{\parnumstyle{(121-123)}}% range of parnums
\ifstr{\headmark}{}{\rightbotmark}{\headmark}% titles in head
}

% Thumbregister
\usepackage{xcolor}
%\usepackage{calc}

% Hyperlinks =========================
\usepackage[%extension=pdf,%
 pdfborder={0 0 0}, colorlinks=true,
 pdfpagelayout=TwoPageRight,pdfpagemode=UseThumbs,%
 plainpages=false,pdfpagelabels=true,%
 hyperindex=false,%
 linktoc=all
 ]{hyperref}
%=================================
% Thumb Register =======================
\usepackage[%plainthumbsoverview, % does not work
thumblink=titleandpage,%titleandpage,  %title, %rule,
linefill=dots,
height={10mm}, %auto
minheight={12mm},%
width={5mm},    %auto
distance={1.5mm},
topthumbmargin={3cm}, % auto = default
bottomthumbmargin={auto},%
nophantomsection=false,
ignorehoffset=true,
ignorevoffset=true,
final=true,%
hidethumbs=false,
verbose=true]{thumbs}%[2012/02/25]% v1.0n

% How to make this better?
\newcommand{\Chapter}[1]{
\chapter{#1}
\addthumb{#1}{%
\space\Large\sffamily\bfseries\thechapter}{white}{black}
}
%==================================
%\makeatletter
%\addtolength{\th@mbwidthxtoc}{-3cm}
%\addtolength{\th@mbwidthxtoc}{-20mm}
%%\setlength{\th@mbwidthx}{3mm}
%%\setlength{\th@mbwidthxtoc}{\th@mbwidthx}% 2cm %default: \textwidth
%\makeatother

%\addtolength{\th@mbwidthxtoc}{3mm}
% \addtolength{\textwidth}{-0.5\textwidth}
%\th@mbwidthx+20pt

\usepackage{mwe}
\begin{document}
\pagenumbering{arabic} %thumbs needs that?!
\addthumbsoverviewtocontents{section}{Thumbregister}  % ``intoc''
\thumbsoverview{Table of Thumbs}

%{\clearpage
%\tableofcontents     
%\clearpage}

\Chapter{Chapter 1}
Theorem of Pythagoras. \marginnote{\parnumstyle{121}} 
\section{Very important section}
\lipsum[1]

\Chapter{Chapter 2}
\marginnote{\parnumstyle{122}}
\lipsum[1-2]

\Chapter{Chapter 3}
\marginnote{\parnumstyle{122}}
\lipsum[1]

\Chapter{Chapter 4}
\marginnote{\parnumstyle{122}}
\lipsum[1-20]
\end{document}