I have only just read this thread all the way through, and the answer for Andy is to ask the caravan manufacturer.
How much value it has is open to debate. But regardless of what the manufacturer states, it is up to the driver of the outfit to ensure it is legally compliant and safe to tow. So that may well mean adding ballast for its first journey.
The point was made that from its first use onwards, the caravan is very unlikely to be as empty as when the manufacture made it. So any published empty nose weight will be of very limited use.
The actual value of the nose load is affected by how the caravan is loaded and how the tow car is loaded, so will change with use of the caravan, so it will be different at the start of a journey compared to the end, even just because of the reduced weight of fuel affecting the suspension of the car, not to mention all the holiday souvenirs and bottles of wine you have collected.
From the above it is important to measure nose load correctly, The measurement must be made with the hitch at the same height above the ground as when the loaded caravan is hitched to the loaded car. Even the weight of the driver will affect the height of hitch. This true for all trailers, but especially so for twin axle trailers which are particularly sensitive to the height of the hitch due to the interaction of the twin suspension units.
A point of law,
There are regulations concerning nose weight. I believe they call for an absolute minimum of 25Kg or anything more than 4% of MTPLM.
The load must not exceed the loading limit of either the tow bar or the trailer hitch
When measuring nose weight (Static vertical hitch load), although I agree in principal that there should be some allowance for the accuracy of a gauge, As far as I know the law does not does not say it makes allows for errors.
On that basis one has to assume the law places an absolute limit, and the user must allow for the error of the measuring device and err on the lower side.
However the device that the authorities might use to check a nose weight will also have errors, but should have a calibration record to show how to compensate for them. Any allowance made for tolerance is entirely at the discretion of the enforcing officer, but may be subject to a guideline from Association of Chief Police Officers (ACPO.)
The bathroom scales are likely to be easier to read, and have finer graduations than the typical sliding cylinder nose weight gauges. Scales are easier to adjust and are likely to give more consistent readings. They can be checked relatively easily by using the fact that 1Litre of water weighs 1Kg, so a lightweight water container filled with a measured amount of water (e.g an Aquaroll) can be used to check some of the scale.
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