[NTG-context] how to use \dimexpr and fractional values

Hi all, long story short: I want to calculate a width or height from user input, which is in the form "0.x" (as in "0.3\textwidth") since I want to use that number differently in different circumstances. Problem is that \dimexpr doesn't like decimal values. Example (uncomment second \externalfigure to see the problem): \starttext \def\myfactor{0.3} \externalfigure[mill][height=\myfactor\textwidth] %\externalfigure[mill][height=\dimexpr10cm*\myfactor\relax] \stoptext (silly, but shows my problem). How can I obtain an image of height "0.3*10cm" from my \myfactor macro? I tried to get my head around \numexpr etc. in the etex manual, but couldn't really figure out what to do. Thanks, and all best Thomas