It's a horrible system...
You're relying on the fit of a case, which I take is assumed to be zero headspace with every chamber and bolt in the cosmos!
Go to 'Woods' posts on both pages for description of a better method (R-P Tool)(with pics):
Reloader's Nest Forum - OAL
Some folks do this with a dowel or cleaning rod, and do yourself a favor and pick up a Sinclair 'nut' comparator.
And oh yeah, drop that Stoney point copy in a red Hornady trash can.