> That is in this case a better solution. For one in my solution the table became to wide. (Do not know why.)

The combined width of the columns in your table had been \textwidth but there is also a space between the columns which makes the table wider than the normal text.

Is it know how much spave this is? For another table I now use:
  \starttable[|w(.10\textwidth)|p(.82\textwidth)|]

This looks about right, but if there is a better way to do it …

Set only a width for the first column, the width of the “p” column

is calculated by ConTeXt.

That does not work. The p column will become as long as the first column:
\starttext

I hadn’t noticed you switched from tabulate to table in your example.

Look into the wiki, maybe you find some info or use natural tables

which don’t use a space between columns.