% from/to list:

\startMPcalculation
    path p ; p = (0,0) -- (PaperWidth,.5PaperHeight);
    path q ; q = (0,PaperHeight) -- (PaperWidth,0);

    passvariable("myheight",ypart (p intersectionpoint q)) ;
\stopMPcalculation

\setuplayout
  [topspace=0pt,
   header=0pt,
   height=\MPrunvar{myheight}bp]

\showgrid

\starttext
    passed value :\MPrunvar{myheight}
\stoptext
