i just rely on experience to judge distance and i always tell myself when drawing a bead ..."if you think he's 50 yards away, let him get closer."
i shoot a 3-inch, 12 gauge and have killed them at 50-55 yards but try not to take those shots anymore.
if they hang up at 50, usually a soft purr will bring him a few steps closer.
but if he's comin, i let him keep comin. i'm addicted to that rush of watching the tom put on his show as he gets closer and closer...