Hello Nick,
A caravan manufacture cannot tell you what the actual nose load of a caravan is, becasue it always depends on how the caravan is loaded at the time. The owner is expected to reposition where items are stowed to trim the the nose load to a suitable value. A brand new caravan straight from the dealer without any of your equipment or luggage may have an excessive nose load. This is not a fault, it just means its designed to rely on your items to address the balance, The caravan may need ballasting to set a suitable nose load when being towed empty.
There is NO single fixed value of nose load you should aim at which is why the industry suggests you trim for about 5 to 7% of the trailers weight.
Very few caravanners know exactly how heavy their caravan actually is, and because the available payload in a caravan is a relatively small percentage of the caravan whole weight, it is easiest to base you target nose load on between 5 to 7% of the caravans MTPLM.
Nose load plays an important role in helping to keep the caravan towing properly. Too little and the caravan can become unstable even at just modest speeds. It is generally better to aim towards the top of the available nose load range, as it increases the handling margins. Your range is the smaller value of either the Cars tow balls load limit (S value) or the caravan chassis nose load limit. In your case Elddis have already worked out the 5 to 7% figures so there is a starting point.
I must point out that caravan stability is not controlled by just one feature such a nose load, it has to be sensible approach to nose loads, and the distribution of weight within the caravan, not forgetting having an adequate car and everything properly maintained and working. Considering road conditions, and not forgetting the driving style.
Don't forget that the actual nose load of a caravan is pushing down on the rear most part of the tow car, and that will apply load on the cars rear suspension. You will need to check your cars specification for details, but some manufacturers do tell you to limit what you carry in the back of a car if you are towing. Your cars data plate tells you the load limits for the rear axle.
Provided you load your car correctly even when towing, you should never need to use spring assisters. The car manufacture will have worked it all out, and if you find your car is too low at the rear it is almost always being overloaded, or something has worn or is broken. Don't forget to check your cars handbook for advice about tyre pressures when towing.
PJL Edit - Opps- Missing important word now added in green .