The fact that ATC requires you to "report established" usually implies that you are within calibration range
Nah it doesn't you can be 40 miles out pointing straight at the runway on a direct arrival to a center fix cleared down to flight levels and they can and have given me that instruction.
You can't capture the glide though until 15miles. But you can still do a CDA mentally until you get there.
One of the joys of flying at 3am when no other bugger is about.