10 \addtolength 这一命令是局部生效的，如何能使其在全局生效?

我希望在titleformat 中的长度设置在titleformat 外也可以生效，该怎么做？代码如下。

\documentclass[b5paper,svgnames,10pt]{book}
\usepackage[utf8x]{inputenc}
\usepackage[title]{appendix}
\usepackage{imakeidx}
\makeindex
\usepackage{tikz,calc}
\usepackage[explicit]{titlesec}
\usepackage{xcolor}
\usepackage{colortbl}
\usepackage[margin=1.5cm]{geometry}
\usepackage{lipsum}
\usepackage{newtxtext,newtxmath}

%%%%%%%%%%%%%%%%%% fancy heading  \chapter %%%%%%%%%%%%%%%%%%%%%%%%
\newlength{\testlen}
\setlength{\testlen}{2cm}
\newcommand*\chapterlabel{}
\titleformat{\chapter}
{\gdef\chapterlabel{}
  \normalfont\sffamily\Huge\bfseries\scshape}
{\gdef\chapterlabel{\thechapter\ }}{0pt}
{\addtolength{\testlen}{1cm}
\begin{tikzpicture}[remember picture,overlay]
    \node[yshift=-3cm] at (current page.north west)
    {\begin{tikzpicture}[remember picture, overlay]
        \draw[fill=LightSkyBlue] (0,0) rectangle
        (\paperwidth,3cm);
        \node[anchor=east,xshift=.9\paperwidth,rectangle,
        rounded corners=20pt,inner sep=11pt,
        fill=MidnightBlue]
        {\color{white}\chapterlabel#1};
      \end{tikzpicture}
    };
  \end{tikzpicture}
}

\titlespacing*{\chapter}{0pt}{50pt}{0pt}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\begin{document}

\tikz{
\draw[blue](0,0)--(\testlen,0);
\draw[blue](0,-2cm)--(3cm,-2cm);
}

\end{document}

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