On 2013–08–26 "H. Özoguz" wrote:

> >\starttext
> >   \spaceskip=1ex plus 1ex minus .2ex
> >   \input knuth
> >\stoptext
> >
> >Marco
> 
> That simply overrides protrusion and justification,

The following text has protrusion and is justified.

\setuplayout [width=6cm]
\definefontfeature [default] [default] [protrusion=pure]
\setupalign [hanging]
\showframe
\starttext
  \spaceskip=1ex plus 1ex minus .2ex
  \input knuth
\stoptext

> forcing a constant distance.

If you provide stretch and shrink the distance is not constant.

> The question is, only to add an minimal distance, wich has not to be
> undercut while protrusing and justifying.

Example please

Marco