18mm flange focal distance ? A matter of gambling ?

Several User Guides for different models of Sony NEX Cameras mention the flange focal distance with "ca." or "about" 18mm. When given in inch, this measurement differs by 0,25mm. When adapting old lenses onto the NEX, very often we see reports about incorrect position on the scale of the lens versus actual distance of the object, maybe leading the producer of the adapter to make it a tiny bit shorter, to be able to guarantee correct focus all over and up to infinity distance. Not only does this worsen the closest position one can focus on, it also makes the scale on the lens useless, and furthermore prevents to adjust the focus of the lens when taking pictures of the moon, stars, etc. Now what is actually the cause of such problem, the camera, the adapter, or more unlikely the lens itself ?
For that reason, I wanted to know - what is the correct and exact flange distance for the NEX camera:
March 11th, 2012 - Asking Sony by using a form on their web site - asking for the exact flange distance of NEX-5N
all following correspondence by eMail:
March 14th - Sony: asking "what model of NEX ?" (method of delaying ?)
March 15th - my reply   (in German and English, as the Sony EU HQ is in U.K.)
March 20th - Sony: internal inquiry needed   (only dummies in Germany Support ?)
May 7th - me, asking for an answer     (yes, I'm still alive, my asking was serious)
May 16th - Sony: your question has been forwarded again to the appropriate department (it's not me, it's them)
May 19th - Sony:
We are sorry to tell you, that we do not have information about the exact flange focal distance.
("Wir bedauern Ihnen mitteilen zu müssen, dass uns das gewünsche genaue Auflagemaß nicht vorliegt.")
After more then two months, they cannot provide such simple und single detail ? Cannot ? Or more likely - do not want to - do not want to admit - the precision is limited by the tooling used, no final adjustments like shimming taking place, not on the camera, nor on the lenses? The correct operation is guaranteed and compensated for by means of electronic control as long as appropriate Sony devices and components are being used....?
Sony - is it sooo complicated to explain that, or what is the problem ? Would it help me learning Japanese ? And what about you - do you know more about this?
My conclusion: The flange focal distance of the NEX' is circa/about 18mm, precision a matter of good luck.