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}
Best Answer
THANKS TO PERCUSSE
this is the answer, is right?