Did you try just putting something like
\spaceskip .5em plus .25em minus .25em
somewhere at the beginning of your document? Short explanation: \spaceskip
is (low-level) TeX parameter responsible exactly for that.
Edit: Notice that em
depends on the current font, so you should first set the font (e.g. by \setupbodyfont
) and only then issue the above command.
Also, strangely enough, after skimming through the ConTeXt (MkIV) sources, it seems to me that ConTeXt does not have a high-level interface for that (\setraggedskips
is not what I'd call a "high-level" interface). This has a benefit of making me quite sure that nothing (e.g. in \starttext
) is going to change \spaceskip
.
Edit: as egreg points out in the comments, changing \spaceskip
is probably a bad idea; it might be better to use \emergencystretch
, which (in case TeX cannot typeset a paragraph within its "badness" limits, even with hyphenation) is added to the "stretch" compoment of \spaceskip
, so it is considered only, well, in emergency (bibliographies or twocolumn layout are two examples when one might want to use it - sparingly, of course!).
Notice that there is also \xspaceskip
, which has a similar application as \spaceskip
, but TeX uses it at the end of the sentence (unless \frenchspacing
is in use).
\setuptolerance[horizontal,space]
.\setuptolerance[space]
only fixes\spaceskip
and cannot change it.\spaceskip
is a good idea to begin with.\emergencystretch
is probably usually the better way to go). (This, either, does not have any high-level interface in ConTeXt, except for\setuptolerance[stretch]
.)