There's a saying that goes something like this...
"There's a simple answer to every question... and it's wrong."
It depends on your preferences to a large degree.
The important thing to realize is that every brand of brass has a different neck thickness.
Also every time you fire and resize your neck, you work harden the brass and that changes the spring back. For this reason your neck tension will change over time and you will need ever smaller bushings to compensate.
You can also anneal the brass from time to time. Even if you do anneal you may induce even more variation in hardness.
For accurate long range shooting I try to reduce the effect of the above variables. I use the least amount of neck tension that I can get away with.
My thinking is that the amount of neck tension variation on light neck tension is less than the amount of variation that's possible with more neck tension.
In the end you will find that you actually need several bushings to cover a range of possibilities. You can start with .002 less than wall thickness x 2 + bullet diameter, but that just a starting point. You will probably need more sizes. I think I have about 8 sizes per bullet diameter.
