Second question first, no that is NOT the way to calculate bushing size. If you have any loaded rounds, you need to subtract .002 from the diameter of a loaded round's neck , over the largest diameter of the bullet. Lacking a loaded round, if you don't have a case with enough neck tension to have its neck enlarged slightly when a bullet is seated, you will have to accurately measure the thickness of a typical neck, double that, add the diameter of the bullet (measured) and subtract .002 from that.
As to setup, there are a couple of approaches that should get you close. First of all, you need to have a way to accurately MEASURE shoulder bump. Hornady makes a tool that is an attachment for calipers. If you are working with ammunition that is loaded relatively hot, or one round for that matter, you can set the die by turning it down till it touches the shell holder, backing it up a quarter turn, and then doing repeated adjustments checking as you go. When you get to the point just before the shoulder in the die is about to start touching the shoulder of the case, your measurement will increase slightly, and then decrease from that point. On the one warm firing, a new case will not come to the full headspace of the chamber, so you can set the die to give the same measurement as the fired case. Or you can just size the neck and load a case 3-4 times with a warm load, and subtract .001 to .002 from what it measures. Just don't forget to reseat the fired primer so that it is well below flush, or use a punch to remove it, before measuring the fired case. Remember that one full turn of a die moves it .071 thousandths. Those fellows that talk about changing the die setting a sixteenth of a turn at a time are telling you to move the die four and a half thousandth at a time. You can do this adjustment without the bushing in the die, to save working the neck too much with multiple insertions into the die, particularly if you are using the die with the expander ball. It is normal to have some variation is shoulder bump at the same setting. Keeping that to a minimum is a whole other subject.
