\showframe

\setupfloatcaption
  [width=max]

\startbuffer
\startnarrower[4*right]
    \startitemize
        \startitem \input tufte \placefigure[effective,here]{}{\framed[width=11cm]{}} \stopitem
    \stopitemize
\stopnarrower
\stopbuffer

\starttext

\getbuffer

\setupfloats
  [location=left,
   leftmargin=\effectiveleftskip,
   rightmargin=\effectiverightskip]

\setupfloatcaption
  [leftmargin=\effectiveleftskip,
   rightmargin=\effectiverightskip]

\getbuffer

\stoptext
