Here's a few things I've learned using the tool....Brass quality is important. Case heads that contain imperfections (out of round, lumps on rim) will give false concentricity readings. When turning in the thumb screw the rotating action of the screw will sometimes turn the case past the 180* point & make concentricity worse. A perfectly concentric cartridge won't always line up at the same graduation point on the dial indicator due to machining tolerances of the bullet spindle. However the amount of R/O will indicate accurately when retested on your RCBS gauge. Quality brass like Norma & Lapua will give faster, more consistent readings. Well annealed brass is much easier to "true". Also, bullets that seat higher than base of neck require less thumb screw force but won't react to truing as well as bullets seated to base of neck. This problem gets worse w/ thin walled brass........ Just a few observations. Hope this is helpful :) :)