Align TikZ grid with geometry crop marks

I am drawing a grid on top of the layout defined with geometry.

\documentclass[letter]{article}
\usepackage{geometry}
\geometry{
showcrop,
paper = a4paper,
layout = a5paper,
layoutoffset={1cm,1cm}
}
\usepackage{tikz}
\usetikzlibrary{calc}
\usepackage{lipsum}% just to generate text for the example

\makeatletter
% Code from:
% http://tex.stackexchange.com/questions/85064/grid-for-textpos-layout
\newcommand\rectangulargrid[2]{%
\tikz[
remember picture,
overlay,
yscale=-1,
xstep=\Gm@layoutwidth/#1,ystep=\Gm@layoutheight/#2,
xshift= -\Gm@layouthoffset, yshift=-\Gm@layoutvoffset,
]
\draw ($(current page.north west) - (\Gm@layouthoffset, \Gm@layoutvoffset)$) grid
($(current page.north west)+ (\Gm@layoutwidth, \Gm@layoutheight) + (\Gm@layoutwidth/#1, \Gm@layoutheight/#2)$);}
\makeatother

\begin{document}
\lipsum[1-3]
\rectangulargrid{16}{16}
\end{document}


I don't know why the grid lines are not aligned with the crop marks.
Any idea?

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please write up your solution as an answer if you have a moment. :) –  Paul Gessler Feb 24 at 14:35

I achieve the same effect drawing the lines myself

\documentclass{article}
\usepackage{geometry}
\geometry{
showcrop,
paper = a4paper,
layout = a5paper,
layoutoffset={1cm,1cm}
}
\usepackage{tikz}
\usetikzlibrary{calc}
\usepackage{lipsum}% just to generate text for the example

\makeatletter
\newcommand\rectangulargrid[2]{%
\begin{tikzpicture}[
remember picture,
overlay,
yscale=-1,
]
\draw[fill=red] ($(current page.north west) + (\Gm@layouthoffset, \Gm@layoutvoffset)$)
circle (5pt);
\draw ($(current page.north west) + (\Gm@layouthoffset, \Gm@layoutvoffset)$) -- + (0,-3cm);
\draw ($(current page.north west) + (\Gm@layouthoffset, \Gm@layoutvoffset)$) -- + (-3cm,0);

\foreach \x in {0,...,#1}
{
\draw
($(current page.north west) + (\Gm@layouthoffset, \Gm@layoutvoffset) + \x*(\Gm@layoutwidth/#1, 0)$)
-- +(0,\Gm@layoutheight);
}

\foreach \y in {0,...,#2}
{
\draw
($(current page.north west) + (\Gm@layouthoffset, \Gm@layoutvoffset) + \y*(0,\Gm@layoutheight/#2)$)
-- +(\Gm@layoutwidth, 0);
}

\end{tikzpicture}
}
\makeatother

\begin{document}
\lipsum[1-3]
\rectangulargrid{16}{16}
\end{document}

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