@DavidCarlisle Since you are an expert in doing things using the `picture` environment, you can do some equivalent to:
\begin{tikzpicture}[remember picture, overlay]
% some extra trickery to ensure that the grid only has whole squares
\draw[step=20pt, gray!40, thin]
let \p1=(current page text area.south east),
\n1={20*int(\x1/20)},
\n2={20*int(\y1/20)}
in (0,0) grid (\n1,\n2);
\end{tikzpicture}%
(https://tex.stackexchange.com/a/562833/7832)
using `picture` environment?