Get position of a tikzmark

Question

In order to solve this question, I'd like to obtain the absolute y position of a tikzmark (actually I want the distance from the border of the text area, but I guess it's not too hard to convert between these two values. The goal is to combine it with this solution). But I can't find how to do, any idea?

\documentclass[a4paper,12pt]{book}
\usepackage{tikzpagenodes}
\usetikzlibrary{calc,tikzmark}
\usepackage{lipsum}
\usepackage{showframe}
\usepackage{everypage}
\usepackage{changepage}
\begin{document}
\noindent Here is some text.
\begin{adjustwidth}{1cm}{-8em}
  \noindent \tikzmark{hello}Hello. \message{Horizontal distance from border is ???}
\end{adjustwidth}
\end{document}

Answer 1

\documentclass[a4paper,12pt]{book}
\usepackage{tikzpagenodes}
\usetikzlibrary{calc,tikzmark}
\usepackage{lipsum}
\usepackage{showframe}
\usepackage{everypage}
\usepackage{changepage}
\begin{document}
\noindent Here is some text.
\begin{adjustwidth}{1cm}{-8em}
  \noindent \tikzmark{hello}Hello. \message{Horizontal distance from border is ???}
\end{adjustwidth}
\begin{tikzpicture}[overlay, remember picture, shift=(current page.north west)]
\draw[red] let \p1=(pic cs:hello) in (\x1,\y1) -| (current page.north) node[right, pos=0.75] {\y1 \xdef\mydim{\y1}};
\end{tikzpicture}
\mydim
\end{document}