The right size pistol case, 9mm, 40, 45 etc to fit onto the neck properly is probably the easiest version of this but if you don't have access to pistol brass then Chuck's system works good too. Just use the right size case so that it sits on the Datum line, roughly 1/3rd of the way down the shoulder. The black line on the shoulder in the picture below, designating where to measure head space, is the Datum line.
