[NTG-context] letter booklet

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