The relation between neck tension (difference in diameter) and how far your can seat into the rifling (past touch) is easy to determine using dummy rounds.
Use cases that are in the exact condition that they would be if you are loading them, sized with different bushings or mandrels. Measure them as loaded with the bullets seated long, and measure them after chambering. If they are shorter after chambering, that length the bullets were pushed back to is old school jam.
I don't anneal, but what I see is the more neck tension the deeper I can seat without the bullet being moved as a round is chambered.
A lot of the confusion is caused because fellows use words without knowing their original meanings.
Back in the day, short range group shooters, who load at the range. Used jam as a reference for where they were seating bullets. ( some probably still do) They would load a round (Remember they were at the range and could chamber it at the line during a firing period.) Measure its length, chamber it, remove it and if it was shorter, that length was jam. Notice, the word was used as a noun. It was a loaded length (a measurement) that a specific bullet would be pushed back to when loaded long, with the case in the same condition that it would be when loaded for a match, same powder fouling, same neck tension. No one was annealing. When asked where they were seating, they would say something like, six off jam, meaning six thousandths shorter than the length that the bullet had been pushed back to in the test. That length was referred to as jam.
If you use a dummy round you eliminate the risk of powder being dumped in the action if the bullet sticks in the barrel as you attempt to extract the round.
I have never heard soft jam, but decades back a prominent high powder shooter explained what that long range shooters in that sport, who were shooting prone with bolt rifles called soft seating. Basically it involved loading long, with light neck tension so that the bullet would be pushed back a little as the round was chambered.
One thing that you did not tell us was if you are satisfied with the accuracy that you are getting, but your question might indicate that you are looking for improvement.
One thing that does not get mentioned much in internet discussions is that different powders "like" different amounts of neck tension. In the 6BRA world, Varget does well with moderate neck tension, while H4895 likes more. In short range group, 133 would prefer .004 while LT32 would be happy with .0015 or so. The obvious way to determine optimal neck tension is to shoot your best load with different tensions.
My suggestion is that you do your own testing, and believe your targets.
-Al
