The 85% suggestions is only advisory it has no legal basis. BUT the principle of keeping the weight of a trailer as small as possible should not be dismissed out of hand.
Caravans by virtue of their size and low density present a difficult trailer as they are very easily affected by the windage from other vehicles, and from just side winds. It is just plain sensible to keep caravans as small as possible.
The previous replies are correct. The legal limit is the tow vehicles Gross Train Weight, which is the maximum coupled weight limit of the entire outfit.
Other clive has already pointed out the likely problem with the load margin of the caravan. Its going to be quite small, and it will mostly be used up with items like caravan battery, pots and pans, EHU lead wheel chocks or ramps, waste and freshwater containers, cutlery, .... the essential living equipment and that's before personal items like clothes boots, bedding, food and drinks, Dare I mention makeup.
There is a possibility you might be able to extend your available capacity in the because of the way the weight and load limits are technically defined. This will only work for you becasue you have a caravan whose MTPLM is greater than the braked towed weight limit of your car.
The MTPLM of a trailer is the maximum limit for the weight of the whole trailer. Importantly this includes the nose load distribution of the trailer.
The cars braked towed weight limit only applies to the load carried by the trailers road wheels, it ignores the nose load. This is becasue the nose load is "carried" by the car and forms part of the cars loading.
Legally you could increase your apparent load margin in the caravan by the amount of nose load you create and apply to the car. The problem is knowing what that value is - which needs to be measured.