We can take off with a zero foot cloud base (fog) and 350m visibility. Provided we have information for RVR from the three transmissometers.
We also have the capability of an autoland to return in a critical situation and the runway is suitable, or the required equipment for a takeoff alternate within 60 mins as per route distance limitations.
I'd think very carefully before launching in your average piston twin with a 300ft base and 2000m visibility.