Every single bit of this is dead wrong.
For one thing, a neck expanding so little as 0.0000000000001" will FULLY RELEASE a bullet.
That is, the bullet is just swinging in the wind, with nothing holding it by then.
ANY neck clearance(which is not interference fit), is enough, and does not increase load pressure.
Next; You only get spring back for grip, and spring back is usually 0.0005-0.001 max.
Any amount of downsizing leaving interference beyond that is no more than excess, and doing nothing more for bullet grip.
JJMoody, you decide what neck thickness and clearance you want. Basically, what you're comfortable managing.
Necks around 12thou works, 1thou neck clearance works. That's just my baseline where I have control over it.
*0.308+0.024 = *0.332 loaded neck OD, subtract 2thou for 0.330 neck bushing, the necks will spring back to ~0.3305" OD. Mandrel neck expand for an interference ID of 0.3065 -0.3070.
*(measure actual bullet dia)
If you want to start at 1/2 tension, neck sizing length can be 1/2 seated bullet bearing length.
This because tension is NOT interference fit. Tension is spring back grip X area gripped (PSI).