Calculating the weight ratio by using the maximum laden weight of the caravan and dividing that by the kerbweight of the car is standard practice. It represents the worst possible condition weightwise when towing. Of course, if you never load anything into the caravan but put everything in the car, the ratio will drop significantly, but the resultant value would be meaningless because it cannot be compared with anything else. A lot of people who tend to make full use of the car's max. permissible GVW and never tow with the car only at its kerbweight therefore choose to go higher than 85% so long as the manufacturer's spec allows.