The gauge I use references off the case body and indicates a little bit back from the bullet tip,, in theory that is what you want. When I started chasing my run out issues I found out that it was all introduced at the bullet seating process . I upgraded my seating die to the Redding competition seater die and runout improved big time,and so did my competition scores.Now I don't even bother checking runout . When a guy straightens a round he changes neck tension . .005 or less is good enough for me to be quite competitive.