The first thing context does is to check the value of width: (the width of the textarea is stored in \makeupwidth, not \textwidth)
1. width=middle
if cutspace=0pt then cutspace=backspace endif
makeupwidth = paperwidth - backspace - cutspace
2. width=fit
when i look at the source i don't know if anyone want this setting because very complicated and the calculation is
makeupwidth = paperwidth - (backspace - leftmargin - leftmargindistance - leftedge - ...) - (cutspace - rightmargin - ...)
3. width=<dimen>
makeupwidth = <dimen>
Example:
\definelayout[1][backspace=4cm,width=12cm] \definelayout[2][backspace=5cm,cutspace=7cm,width=middle] \definelayout[3][backspace=3cm,cutspace=0pt,width=middle] \definelayout[4][backspace=4cm,width=fit] \definelayout[5][backspace=4cm,leftmargin=3cm,width=fit] \definelayout[6][backspace=4cm,leftmargin=4cm,width=fit] \definelayout[7][backspace=4cm,leftmargin=2cm,width=fit] \definelayout[8][backspace=4cm,leftmargin=2cm,rightmargin=3cm,width=fit] \definelayout[9][backspace=4cm,leftmargin=2cm,rightmargin=4cm,width=fit] \definelayout[10][backspace=4cm,leftmargin=2cm,rightmargin=2cm,width=fit]
\showframe
\starttext
\dorecurse{10} {\starttabulate \NC backspace \EQ \PtToCm\backspace \NC\NR \NC textwidth \EQ \PtToCm\textwidth \NC\NR \NC cutspace \EQ \PtToCm\cutspace \NC\NR \stoptabulate \page}
\stoptext
Wolfgang
to wikify ... where ? -- luigi