We at GR technologies, provide you, the player with two cup diameter measurements; r2 and r4.
r2 is the measurement which corresponds with the Series number of each mouthpiece.
For example, the 62 Series measures .620" @ r2. This is where the rim ends and the cup begins.
r4 measurement is taken at .032" down from the rim plane. This measurement indicates the general feel of the cup diameter.