The Neck Die Bushing fits in the Neck Die and sizes the case neck to grip the bullet.

To calculate bushing size... Measure the diameter of a loaded round where the bullet sits in the neck (Ex. .268).  Now subtract .002 to .003 from that number.  

.268 -.002 = .266 Bushing

Another way is to measure your wall thickness and multiply by 2 and add bullet diameter.  Then subtract .002 to .003 from that number.

.012 Wall Thickness x 2 = .024

.243 Bullet Diameter

Example:  .243 + .024 = .267   Now subtract .003 (.267 - .003 = .264 Bushing)