The setting plug checks the pitch diameter of the ring and contacts the flanks of the thread to the maximum major diameter of the screw.
The major beyond this is to be cleared in the ring. As a positive check on this, modern practice is to use the truncated type setting plug in which a portion of the major diameter has been reduced or truncated.
In use, the thread ring gage is set to fit the pitch diameter on the truncated portion of the plug, then carefully screwed over the full form portion.
If the ring gage has proper clearance in the major diameter, there will be little or no difference in the feel of the ring as it passes from one section of the plug to the other.