[NTG-context] How can I use a variable for a MetaPost coordinate ?

["peter.hopcroft--- via ntg-context" <ntg-context@ntg.nl>]

Hello list,

I want to draw lines on a page. I want to pass my line coordinates as a percent of \width or \textheight, rather than as millimetres. So that if I change the page size I don't need to change the coordinates. 

The MWE below compiles but the pdf does not have any lines drawn on it. 

Is there a way of doing calculations on the coordinates for the draw using ConTexT variables like \textheight?

Thank you,
Peter Hopcroft

------

\starttext
Some text

\startMPcode
	pickup pensquare scaled 1pt;
	draw (0mm,0mm)--(0mm,\textheight) withcolor red 
\stopMPcode

More text

\startMPcode
	pickup pensquare scaled 1pt;
	draw (0mm,0mm)--(0mm,50mm) scaled \numexpr\textheight/50mm\relax
\stopMPcode

\stoptext


------

___________________________________________________________________________________
If your question is of interest to others as well, please add an entry to the Wiki!

maillist : {listname} / https://mailman.ntg.nl/mailman3/lists/{list_id}
webpage  : https://www.pragma-ade.nl / https://context.aanhet.net (mirror)
archive  : https://github.com/contextgarden/context 
wiki     : https://wiki.contextgarden.net
___________________________________________________________________________________