Mavic supplies the spoke support diameter of all of their rims at there website: www.mavic.com. The Bicycle Forest has added 3mm to these values to arrive at the effective rim diameter. This has proven to work for us but the user can feel free to adjust the numbers as they see fit.
Be aware that to enter a different ERD, one should set the rim choice to "Custom" so the program doesn't revert to its stored values for the given rim.