箭头框对齐

Question

我必须在下图中垂直对齐“箭头框”？所以我有两个问题：

1. 如何读取“inner sep”寄存器值和边界以及应用样式的连接线厚度寄存器值？
2. 有没有什么方法可以全局调整“箭头框”中箭头尖端与尖线边缘的距离？

梅威瑟：

\documentclass{standalone}
\usepackage[utf8]{inputenc}
\usepackage{pgfplots}
\usetikzlibrary{plotmarks}
\usetikzlibrary{arrows.meta,bending}
\usetikzlibrary{shapes}
\usetikzlibrary{positioning}

\usetikzlibrary{backgrounds}            %| for arangement objects on layers
\pgfdeclarelayer{myback}                %|
\pgfsetlayers{background,myback,main}   %|

\begin{document}
\def\mtw{3.0}       % box width
\def\pal{2.5}       % perturbation arrow length
\def\ptw{4.0}       % side box width

\begin{tikzpicture}[
    line join=round,
    every node/.style={font=\footnotesize,align=center},
    process/.style={text width=\mtw cm,fill=white,draw},
    perturbation/.style={draw,right=1pt,arrow box,arrow box arrows={west:\pal cm},thin,text width=\ptw cm},
    node distance=8ex,
    flux/.style={very thick,-{Stealth[flex=1.2]}}
    ]

    \node (n1) at (0,0) [anchor=south,process] {initial conditions};
    \node (n2) [below=of n1,process] {first stage of the process};
    \pgfmathparse{(\pal cm-0.5*\mtw cm)}        % shorter arrow length
    %\pgfmathparse{(\pal cm-0.5*\mtw cm-0.3333em)}  % shorter arrow length; 0.3333em is "inner sep" value
    %\pgfmathparse{(\pal cm-0.5*\mtw cm-\mydist cm)}    % shorter arrow length; how to put into \mydist "inner sep" and line widths?
    \edef\pas{\pgfmathresult pt}
    \begin{pgfonlayer}{myback}
        \node [perturbation,right=1pt of n1.east,arrow box arrows={west:\pas}]
              {perturbations of initial conditions};
        \draw [flux] (n1) -- (n2)
              node [midway,perturbation] {perturbations of external conditions};
    \end{pgfonlayer}
\end{tikzpicture}
\end{document}