I use the darkened room, cake pan of water method and have had no problem. When I start to see the flame off the top of the case start to change it's hot enough. It's pretty subtle, but I get very consistent results. I also disagree that you can't be as consistent by eye as you can with an aid. I used to be a glass blower (scientific glass) and had to use a temperature device called a Pyrometer that heated a wire to a calibrated temperature you viewed against the hot object. With experience once could reach a target temp within +- 10 degrees C. If you are running to the same point visually with all your brass it's going to be pretty consistent.
My own experience with the 45/70 confirms a measurable increase in accuracy and lower SD in velocity by annealing in this manner.