[Exim] What's a reasonable value for message_body_visible?

We raised the default value of message_body_visible from 500 to 5000 when
putting in the system_filter to scan for windows executables a while back.

It spotted a Navidad.exe recently but not yesterday; when the attachment
came about 45k into the message. Obviously I am thinking of increasing
this limit but don't like the idea of slugging a nice effecient mta by
forcing it to read every message (after all, the users don't bother!).

So I'm looking for that wonderful yardstick, "what most people use"!

Is 50k reasonable/ How about 500k, 5M, etc?

I guess what makes navidad different is that it replaces attachments
anywhere in the message, rather than very near the beginning ...

Any thoughts welcome, thanks in advance,
Steve Platt