Going with helotaxi here. To set my full length die I use a lee cartridge headspace guage. I check the fired case and write that size down in a log book. I then start working the die down towards the shell holder sizing the brass. at first the headspace grows as Helo said then you start bumping the neck down. I stop when I get to .000 to -.001 from the original dimension. The brass shrinks a touch when it cools so .000 will give you room to close the bolt.