If I am remembering correctly, the documentation has \setupcolumnsetlines
before \starttext.

Sytse Knypstra <sytse.knypstra@home.nl> schrieb am Sa., 17. Aug. 2019,
12:59:

> In an effort to balance columns in a columnset layout, I found that in the
> following minimal example this effect is not obtained.
> After commenting out the first three lines after \starttext the result is
> ok.
> I am wondering what I overlooked this time.
>
> \definecolumnset[Two][n=2]
> \starttext
>    \startcolumnset[Two]
>       \input knuth
>    \stopcolumnset
>    \page
>    \setupcolumnsetlines[Two][1][1][9]
>    \startcolumnset[Two]
>       \input knuth
>    \stopcolumnset
> \stoptext
>
> Thank you for your suggestions,
>
> Sytse Knypstra
>
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