Everyone else has provided great advice on settings, but I would like to touch on technique. If you are new to the Minelab FBS series of machines, then I recommend a "low and slow" sweep style. Unlike other brands and machines, where a moderately fast sweep will net you better depth, the FBS machines do better with a slow, steady pace. Think of the tortoise in that classic race against the hare: slow and steady wins the day. A faster speed with the CTX will not necessarily result in a missed target, but the TID is much more accurate with a slow, low swing in terms of the actual target and the target depth.
Try a few sweeps in your test garden to get the picture. I have found that, on average, the TID number is off by about 3 or 4 on the CO number, and depth is innacurate by about 3 inches with a fast swing.
The FBS series of machines are super at sniffing out finds in trash and deep finds at solw sweep speeds. You wouldn't want to miss that sweet 1800's silver at 7" in iron, would you??