I don't know why folks want to still try judging "... as as it turn some color..." even in a dark room.
Even when one doesn't overheat it, there's no consistency to neck tension after that -- your eyes
just aren't that good.
Please get the temp stik mentioned above, or Hornady's Annealing kit -- or just put some 750°F
Tempilaq inside a couple of necks until you get the timing down for THAT batch with THAT flame
at THAT distance