[NTG-context] letter booklet

[John Kitzmiller]

Desired outcome: a multipage booklet on letter size paper with margins lined up when duplex printed and folded.

Using imposition with [letter][letter,landscape] did not work, so to start I used:

\definepapersize[booklet][width=140mm,height=216mm]
\setuppapersize[booklet][letter,landscape]
\setuparranging[2UP,doublesided]
\setuplayout[width=115mm,
  leftedge=0mm,  
  leftmargin=0mm,
  backspace=15mm,
  rightmargin=0mm,
  rigthedge=0mm]  
\showframe
\starttext
\dorecurse{8}{\input Ward \page}
\stoptext

but the inside and outside margins do not line up. Where is the elegant path?

Thanks, John