OP - You seem to be an analytical person, time to put on your thinking cap. There is case runout and bullet runout, their causes are different, and need to be addressed differently. Standard FL dies with expander balls can lead to case runout as the expander ball can "walk" as its being pulled through the case. This is a complex phenomenon similar to buckling, not easy to correct - neck turning may or may not help. Avoiding the expander ball solves this problem, with methods described in the previous posts.
What neck turning does is to allow for constant neck tension, assuming the balance of your reloading technique is up to par. Neck tension is one of the many variables that can affect precision ( group size) Understanding the relative dominance of all the variables is most important.
EG - Uncertainty analysis can be used to model a shooter and rifle performance. A shooter is given a perfect gun (eg laser) and manages to shoot 1 inch groups at 100 yards. He can declare himself to be a 1 moa shooter. He is given a gun that shoots 5 moa when shot in a vice. What will be the result? Hint - the answer is based upon the sides of a triangle. In this example, the governing uncertainty is the rifle. Whether the shooter has a good day or a bad day, the result is gonna be close to 5 moa.
You can break this down into all the reloading variables - powder charge and type, bullet weight, bullet geometrical variation, bullet jump, runout, neck tension, etc. What the reloader has to figure out is which of these is the governing uncertainty, and reduce these to manageable levels. Over the years, a fella can get a pretty good understanding of where to prioritize their efforts. Generally, the major uncertainties are:
1) bullet type and weight
2) powder type and charge
3) reloading technique consistency and repeatability. (Making a batch of identical cartridges)
The point of this long winded diatribe is - focus on the big ticket items. If your powder charge is messing with your barrel harmonics and contributing to 3 moa of uncertainty, playing around with lesser uncertainties such as neck tension is essentially useless. As stated in one of the posts above, learn to get good before you get fancy. Set aside neck turning for another day.