The gauges are designed for relative measurements (difference in two measurements) not for empirical data. They COMPARE measurements, hence the term "comparator."
You take a known (Your once fired brass, your bullet in your chamber pushed out to just contact the lands) and measure it using the comparator. Now you have YOUR measurement with that bullet of the length from base to a point on the ogive when touching the lands in your rifle. This is measurement 1.
Say you want your bullets to be 0.020" off the lands: You seat another bullet and measure the same thing as above...only you you keep seating the bullet deeper until your measurement is 0.020" shorter than measurement 1. In theory your bullet is now seated 0.020" off the lands.
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