This is not headspace. Head space is where your shoulders contact the chamber. You need some go-nogo gauges for headspace.

For max COL, I use a stoney point tool, (now made by hornady) and use the split case method.

I now just use split case method. I get tired of buying modified cases by stoney point or sending my fireformed cases back.

Split case, you take a dremel tool and cut the neck, and a tad bit of shoulders down teh case. Now you can bend the neck to hold a bullet. Not too tight, but tight enough to move the bullet. Insert a bullet very long, chamber, and carefully remove case and measure COL. Do this several times.

You also want to do this with a fireformed case for accurate results.