Buffalobobs method is a very good way to do it it IF you cannot measure where it is at.
However if you want to measure and keep track you have a couple options.
You can use either the Hornady Lock N Load headspace tool at Sinclair International - Specializing in products designed for precision rifle ammunition reloading and shooting
part # HK-66 or a custom made bump guage.
Either allow you to measure the fired headspacing and the exact spacing needed to bump back the shoulder .001-.002.
A bump guage is about $20 from a smith and is nothing more than a trued barrel stub that is reamed with a reamer and sits down over the case and measured with calipers.
the hornady tool has a 5 piece bushing set that covers almost all cases.