\def\cals@width{200pt} \cals@halfWidthToDimen0 \cals@width \PackageInfo{test}{half of 200pt: \the\dimen0} \cals@halfWidthToDimen0 {0pt} \PackageInfo{test}{half of 0pt: \the\dimen0} \cals@halfWidthToDimen2{100pt} \PackageInfo{test}{half of 100pt: \the\dimen2}