ColorSource cyc adapter distance from cyc

The MyETC photometrics app shows the ideal distance for a smooth cyc in my application is 2' 9".  Where is that distance relative to the cyc?  Is it the center of the c-clamp?  The front edge of the cyc adapter?  Center of cyc adapter?  Middle of the instrument?  Somewhere else?  The adapter needs to be placed at an angle of 70 degrees, so that changes the distance as well.  According to the app, even a one inch difference can make a noticeable difference.