OK, I have been watching vids and reading up on this and it seems that everyone does things their own way.... It seems that one could use one of the various COAL gauges either home made or commercial and get the COAL using ANY bullet. Once they have that done then using a bullet comparator gauge like the one from Sinclaires to find the Ogive you should be able to find the cartridge length to the Ogive for your rifle.
Once you know the COAL measured to the Ogive for your rifle it seems like there would be no need to test each different bullet in the rifle with the COAL gauge. Can't you just load any new bullet to the same COAL measured to the Ogive?