Originally Posted by FUBAR
I was assuming he was using his bolt, if he does then he still has a head space distance in the measurement and if he doses not use his bolt, different types of projectiles will give him different measurements.
I missed your point, but see what you're getting at now with the ejector plunger forcing the cartridge forward in the chamber vs other tools that don't even use the bolt.
The good news is that you don't have to have a precise measurement. You just need a relative starting point and safe load workup to find the seating depth that works best for your rifle/load.