Why doesn't this work? I use Windows, TeX Live 2012 and pdfLaTeX
\documentclass{article}
\usepackage{calc}
\newlength{\perclenght}
\setlength{\perclenght}{%
\dimexpr%
((((336pt*675pt)*(100-7.5))/100)/\textwidth)-\textheight%
\relax%
}%
\begin{document}
\the\perclenght
\end{document}
calc
does not allow multiplication of lengths. In other words: it does not work because it is not featured.336pt*675pt
would be an area, not a length.\perclengHT
with the typo (it should belengTH
).\dimexpr 10*5 pt\relax
is invalid but\dimexpr 5pt*10\relax
is valid, as the dimension has to 'come first'.