True headspace is the measure of the chamber of your rifle. And use go/nogo gauges with shims.
But you are probably talking about measuring the shoulders of your fireformed case and setting your seater?
I have stoney point (now hornady) head and shoulders gauge.
I myself will not partial full length. I find it can cause other issues like inconsistent shoulders, and runout. I usually customize the die for the rifle chamber if its that far off.