controlling title head distances when grid is enabled

[Csikos Bela <bcsikos425@freemail.hu>]

Dear context users:

I would like to typeset my documents to gridlines. If grid is enabled,
the distances between titles (section, subsection,subsubsection heads)
and text are too big. Here's an example:

\setuplayout[grid=yes]
\showgrid
\starttext
\section{Section One}
\input tufte
\subsection{Subsection One}
\input ward
\subsubsection{Subsubsection One}
\input tufte
\section{Section Two}
\subsection{Subsection Two}
\input davis
\subsection{Subsection Three}
\input dawkins
\stoptext

In the output there are 3 empty lines before and 2 after Subsection One.
It would be enough 2 and 1 lines, respectively. On page 2, between
Section Two and Subsection Two heads 4 empty lines are inserted,
which is to much (ugly). How can I control the empty space before
and after heads when grid is enabled? 

I tried this:
\setuphead[section][before=\vskip-2.8ex,after=\vskip-2.8ex]
but it removes all spaces not only 1 line.

I also tried 
\setuphead[section][before={\blank[2*line]},after={\blank[line]}]
but it doesn't have any effect.

Thank you in advance,

bcsikos

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