In the code below, the rulers are positioned where I want them to be, that is 19pt to the left/above the body. But I would like 0, on each of the h and v rulers, to coincide with the top-left corner of the body. \fgrulerstartnumh{-19pt} seemed like the way to do it, but it only accepts positive integers. OTOH, if I use the []commented[-out] \usepackage instruction, I get the second requirement, but not the first. Is there a way out of this dilemma?
\documentclass{article}
\usepackage[showframe]{geometry}
\usepackage[type=none]{fgruler}
%\usepackage[type=upperleftT,unit=in]{fgruler}
\usepackage{xparse}
%https://tex.stackexchange.com/questions/588521/shifting-the-starting-number-of-a-ruler-such-that-0-aligns-with-the-top-left-cor
\ExplSyntaxOn
\makeatletter
% Adapted from:
% https://tex.stackexchange.com/questions/201488/how-to-get-the-active-left-and-right-margins-to-use-them-in-a-tikzpicture
\dim_new:N \erw_Gmlmargin_dim
\dim_set:Nn \erw_Gmlmargin_dim{\dimexpr\Gm@lmargin}
\dim_new:N \erw_Gmtmargin_dim
\dim_set:Nn \erw_Gmtmargin_dim{\dimexpr\Gm@tmargin}
\makeatother
\ExplSyntaxOff
\begin{document}
\ExplSyntaxOn
\noindent
\verb|\erw_Gmlmargin_dim|~
\dim_use:N\erw_Gmlmargin_dim;~
\verb|\erw_Gmtmargin_dim|:~\dim_use:N\erw_Gmtmargin_dim.\par
%\dim_new:N \erw_hstart_dim
%\dim_set:Nn\erw_hstart_dim{\dim_eval:n{-1 * 19pt}} % ERROR
\dim_zero_new:N \erw_hstart_dim
\dim_sub:Nn\erw_hstart_dim{19pt} % Workaround
\noindent{}\verb|\erw_hstart_dim|~\dim_use:N\erw_hstart_dim;~
\dim_zero_new:N \erw_vstart_dim
\dim_sub:Nn\erw_vstart_dim{19pt}
\noindent{}\verb|\erw_vstart_dim|~\dim_use:N\erw_vstart_dim.\par
\dim_new:N \erw_hshift_dim
\dim_set:Nn \erw_hshift_dim{\erw_Gmlmargin_dim}
\dim_add:Nn \erw_hshift_dim{\erw_hstart_dim}
\noindent{}\verb|\erw_hshift_dim|~\dim_use:N\erw_hshift_dim;~
\dim_new:N \erw_vshift_dim
\dim_set:Nn \erw_vshift_dim{\erw_Gmtmargin_dim}
\dim_add:Nn \erw_vshift_dim{\erw_hstart_dim}
\noindent{}\verb|\erw_vshift_dim|~\dim_use:N\erw_vshift_dim;~
% \fgrulerstartnumh{\erw_hstart_dim}
% \fgrulerstartnum{〈num〉}
% The〈num〉is a nonnegative integer, which will be the starting number on the horizontal and verticalrulers. Default:\fgrulerstartnum{0}
\fgruler[in]{upperleft}
{\erw_hshift_dim}
{\erw_vshift_dim}
\ExplSyntaxOff
\end{document}
