Nono, what I mean with working areas is effectively the area illuminated by the feed-horn, is we had a transmitting LNB attached to it.
This may be different from the corrugated plastic areas (as in your photo), and also different from the metal mesh/layer embedded in the dish.
It would be very intersting if we could get @Trust1 to mount his circular-laser device on the CM120, and simulate a 78* feedhorn on the reflector.
I blieve that this would show where the "ideal" working areas is according to the design of the dish.