\magnification=\magstep3
\hsize=210truemm\advance\hsize by-2truein
\vsize=297truemm\advance\vsize by-2truein\advance\vsize by-24pt
\null
\vskip 10cm
\line{\hbox to 3cm{\hrulefill} 3cm rule \hfill }
\bigskip
\line{\hbox to 3truecm{\hrulefill} should be 3truecm rule\hfill}
\bigskip
\dimen0=3cm \divide \dimen0 by \the\mag \multiply \dimen0 by 1000
\line{\hbox to \dimen0 {\hrulefill} actual 3truecm rule\hfill}
\end