Only two instances that I ever have to turn necks. I have a custom cut chamber on a 6mm 30-30 AI Contender barrel that requires formed brass to be turned and I turn necks on .223 brass that I make .221 FB out of.
A good tubing micrometer is handy for reference and to check concentricity.
In the case of the 6mm I have a known chamber neck dimension. I get about .002 " neck tension with sized brass and a loaded round gets a release of .0015" to .002" clearance when fired.
In the .221 I used the "trial and error" method to get the round to chamber and get the release I was looking for.....neck tension is whatever it is.