The way I setup my FL dies for my rifles is this. I use a fired case from the rifle, I punch out the primer with a Lee universal depriming die and then I use a Hornady Headspace Gauge attached to my calipers to get a measurement to the shoulder. I record the measurement and then I setup my FL die to bump the shoulder on the case back .001 to .002. This is for a bolt action rifle of course, with a semi auto you should bump .003 at least. I find with .001 to .0015 in my rifles I get good chambering and good case life.