You measure the neck of a case with the bullet seated. Then deduct .002 for the neck tension. The bushing size depends on the brass you are using. As an example, I measured a factory Hornady 6.5 Creedmoor case with the 147 gr. ELD Match bullet seated. The loaded round measured .290 at the neck, and after firing it was .294 inches. If I reload the Hornady, it will get resized with a .290 first and then a .288 or .289 if I want even less neck tension. I would standardize on Lapua brass if it is available, but be aware there are small primer and large primer versions of Lapua. I chose the large primer option.