Trying to set  %response.ContentLength in the OnPreHTTP() did NOT work...
Also part of the problem is that some responses has the response header "transfer-encoding: chunked"

which prevents having ContentLength anyway, but that is a different problem. 
The %resposne.ContentLength doesnt change the actual value

so what would be the correct way to achieve this?

in your snippet, myTable(name, counter)
name and counter are the fields you want to look out in myTable?

and thank you for your answers!