[NTG-context] table-caption: centred or not??

[Peter Münster]

Hello,
how are the rules, that determine if the caption is centred?
Here my test-case, "ok" means: this is how I like it and "not ok" means, I
don't like it:

\setupcaptions[width=\makeupwidth]
\def\MyTable#1{\placetable{\dorecurse{#1}{bla }}{\starttable[|c|]
  \NC This is a table.\NC\AR\stoptable}}
\starttext
\MyTable{2}  %     ok
\MyTable{10} % not ok
\MyTable{30} %     ok
\setupcaptions[align=middle]
\MyTable{2}  %     ok
\MyTable{10} %     ok
\MyTable{30} % not ok
\stoptext

What should I do, to get 3 times "ok" ?

Cheers, Peter

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