Annealing question

[centerfire]

In your opinion, at what distance does neck tension start to be a factor, say for the Grendel, if a specific cartridge matters? Past 300, 600, 800?

You'll see it immediately. Frankly even the best new brass with perfect annealing lines still seems to have variable neck tension. Recently I was working with some brand new Lapua 308 Winchester brass and I got so sick of the oxide layer and weird neck tension. I annealed all of the brass over and then tumbled off the lines. Just because brass is high quality and new doesn't mean it's hardness will be uniform. Also, if you wet tumble with steel media too long you'll surface harden the brass.