\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}
