\documentclass[a6paper]{ltjsarticle}
%\documentclass[paper=b5j]{jlreq}
\usepackage{luatexja-adjust}
\ltjenableadjust[profile]
\usepackage[pdfbox]{gentombow}
\begin{document}
\noindent\makeatletter
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aiueo \strip@pt\dimexpr0.35145980351459803\paperheight mm%

\end{document}

