Allowed SMTP connections

[randee]

/etc/exim.conf (mine) has the following lines:

smtp_connect_backlog = 450
smtp_accept_max = 850

Is there a rule of thumb as to what numbers to use here?
For example, if you have 4000 email accounts on the server, set it to ???
(I'm only looking for a rough idea, I realise that would be LOTS of exceptions to any 'rules')

We have been under a DoS attack and during that time, I had to turn up the accept_max numbers so my clients wouldn't get the error message of "too many concurrant smtp connections"

Now that the DoS attack is over, I'm trying to fine tune some things. I can do the time-honored 'try it and see', but was hoping someone has some experience with this.

thanks for any help/suggestions...