Several contributors have raised detail points that shows how inept the conventional towing ratio calculation is at representing the actual situation.
However in the absence of a more refined and realistic method being adopted by the industry, we should continue to use:-
100 x Trailer MTPLM / Car kerbweight Percentage. It is at least a consistent and unvarying procedure.
The oft quoted 85% should not be considered a hard and fast 'Target' as a few percent either way is unlikely to case a major change in the handling characteristics of the outfit.
But of equal importance it must be understood that working to the guideline offers no guarantee of an outfit being legal or safe.
With regards to the actual load you put in the caravan, Most people under estimate the total weight of items they add to the caravan. Don't forget things like, the wheel chocks, EHU cable, water and waste containers, bedding, torches, toilet chemicals, boots and shoes, radio & gadgets, crockery & cutlery, kitchen utensils, especially food items and the little knick-knacks like the matches, pens, pencils, maps, guides, magazines, binoculars, cameras, jigsaws and games, tissues, etc etc etc. Probably more so with families than just couples, I suspect that many caravans are actually overloaded.