I generally try to stay out of these annealing threads because they often are asking questions about the machines which I don't use . I do use the method the OP is using with the temperatures he's trying so I'll chime in .
I've done a good amount of testing on this method and the only way to use 750* is inside the neck as shown above . The marks on the outside of the cases is 450* 1/4" to 1/3" below the shoulder and that is my desired method . As stated in another post cleaning the insides of the necks is a pain .
Yes I also use junk brass to get my timing down but it must be done every time you start a new batch or even if you stop and come back to it . This is because each time you start the flame on your torch the setting will be different resulting in a different flame temperature which changes the timing . I even go as far as if I'm annealing 500+ cases . Every 20 or 30 cases I apply tempilaq to keep me honest .
As for the 750* melting , it does not melt the same as the 450* . The 450 clearly melts and turn translucent while the 750* kinda just turns a different color , It's hard to explain what you see but over time you get the hang of it . Basically if you see any change in the 750* , that's what you're looking for .
How long should the case be in the flame ? I can say with almost 100% certainty that 10sec is way to long . With a standard propane torch the timing is generally about 4 to 6 sec . I have found that at about 10 sec the necks are glowing red or very close to it which is to hot and you made the case neck what I call dead soft . This results in low bullet hold and excessive sizing from head to datum with your standard die setting . Meaning with a dead soft neck and shoulder and your normal .002 bump setting on your die , you will be bumping .005 or more because the case is so easy to size with very little spring back .
As for can you heat up your case enough to get the head of the case to soft . Short answer is yes , long answer is no not even close . The cases above are the result of a test of me trying to do that same thing . I kept the propane torch on the neck and shoulder of the case until the 750* melted below the shoulder . The 750* did not melt until 10+ in the flame and the neck were glowing bright red . As you can see the 450* temp indicator at the web did not even melt meaning even when the necks were completely over annealed and basically trash the web still did not get hot enough . My conclusion was if the heat is applied properly in the correct place , you can't get the head to hot with out ruining the case as a whole .
I also tested different flame temperatures along with MAPP gas which burns much hotter then propane . Top flame is propane , bottom flame is MAPP gas . I have several photos of different durations in the flame for both along with the torches turned up and or down changing the heat of the flames . Long story short is that I found MAAP gas to be to hot for this method ( torch and socket ) because it heated up the brass to fast and my hand eye coordination was not fast enough to pull the case out of the flame fast enough before I felt the temperature of the case neck went higher then what was intended . I think MAPP could be used with a machine that could control the timing better then me watching for the temp indicator to melt then pull the case out of the flame .
Well It's late and I feel like I'm rambling . Hope that helps , I'm sure I left some stuff out . Feel free to ask me any questions I might not of answered or to clarify something .
MG