Decide what you want the inside diameter to be. Let’s say .262 .2625 .263 are diameters or “neck tension” you want to try.
Go to Amazon and order 3 pin gauges. One each of those diameters.
Then get mandrels in all three plus/minus another .001 or so. So, .261, .2615, .262, .2625, .263, .2635...