Re: Neck bushing question...
First Look at it in terms of neck clearance.
what is the loaded neck diameter, of unturned brass, with bullets you intend to use?
For example, if your bullets are true .308", and your neck brass measures 13thou thick, then:
.308+(.013x2) = .334 loaded neck diameters, fitting in a .336 chamber neck, = 2thou of clearance.
You can just seat a bullet & measure with calipers to check.
Still with this example, if clearance is actually 2thou, fired brass will spring back ~1thou from the .336 chamber to ~.335.
You only need 1thou under loaded neck diameter(.334) to grip bullets well, so you downsize necks from ~.335 to ~.332 and they'll springback to ~.333, giving you ~1thou of tension.
The bushing is .332, if you want a little more tension go .331, I wouldn't go more than that.
That is only for this example.
But you can apply the jest of it for what your actual loaded necks measure -vs- fired neck.
Where it turns into trial & error is when your bushing has to size more than 5thou from neck diameters as they come smoking out of the chamber. Such a condition is common, but sets up an excess angle of sizing for the bushing, causing a bit more sizing than predicted(like an extra thou or two). People often end up buying a couple bushings, collect them, they're useful in the future..