The center of the gauge is used to measure for the correct rivet length prior to setting.  There are four notches in the center.  The width of these notches is not a gauge, only the height.  The height on the notch is 1.5 times the width of the rivet.  This should be measured from the back side of the material to the end of the rivet.  For a dimpled rivet the gauge is used from the bottom of the dimple to the end of the rivet.  As shown in photo A the rivet is too short to make a proper shop head, in photo B the rivet is too long and will most likely fold over while setting, photo C shows the correct rivet length.  Using this measurement the rivet should at least meet the gauge.

 

The holes in the ends of the gauge are used to measure the shop side of the rivet after the rivet has been set.  The hole is used to measure the minimum diameter of the shop head.  The rivet tail should either not go through the hole, or should just pop through, but not have any side play.  If the tail goes through the hole as in photo D it is not wide enough to be a structurally sound rivet.  If it does not go through the hole as in photo E the rivet is wide enough. 

 

The notches in the corners are used to measure the minimum height of the shop head of the rivet.  If the rivet is too thin there will not be enough material to make the rivet structurally sound.