Caravanning Now - What's the correct height for my Tow ball?

A few pointers, but it's all a bit vague.

The Caravan club handbook and EC Directive 94/20/EC.

Both the Caravan club (handbook page 640 - 1999/2000 edition) and EC Directive 94/20/EC say that the coupling ball is 50 mm diameter and when measured from ground to ball centre should be between 350 and 420 mm (vehicle laden).
This gives nearly 70mm (nearly 3 inches) of latitude in height.

NOTE: Laden is not specified what Laden means. Is this fully laden as per vehicle manufactures definition or with the passengers (at 68.5 kg per passenger) and 6.5 kg of luggage per passenger.
See also the Witter Trade article here (at time of writing) which states (and I quote):
" What height should the towball be?
The million Euro question! The EC Directive 94/20/EC states that the towball height should be between 350 and 420mm with the vehicle in the ‘laden’ condition. Laden condition may have been defined (by the manufacturer) as either fully laden (the maximum technically permitted mass of the vehicle as shown in the vehicle handbook), or, conventionally laden (defined as 68.5kg per specified passenger seat and 6.5kg of luggage per specified passenger and located in the centre of the luggage storage area). Alternatively, the towball may be at the height or height range specified by the vehicle manufacturer in the homologation documents and possibly the vehicle handbook. Not so straightforward!"

NOTE 2: If the car is actually at its fully laden weight (GVW) then you would exceed the GVW by the weight of the down force on the tow ball. (Thanks to Lutz Schelisch for pointing this out.)

NOTE 3: It's actually far more complicated.
You have to take max axle weight into account and also the overhang length.

For Example: A 75kg caravan nose weight on a 110 LandRover puts around 125kg on the back axle.
Since most cars carry most of their luggage behind the rear axle, it's pretty easy to exceed the rear axle max weight and remain in the gross vehicle weight.
75kg on a 110's tow bar actually takes ~50kg off the front axle.
Luggage behind the back axle also transfers weight from the front axle to the back axle, exacerbating the problem.

See also the page on what weight caravan you can tow