85% rule is to be ignored and the vehicles handbook is where to find out the legal maximum towing limit.My car weighs 1040kg yet i can only tow 750kg braked so if i assume i can tow 85% that would give me a limit of over 850kg but i would be uninsured and towing illegally.Some vehicles can also tow in excess of their kerbweight a Mondeo 2.0tdci can tow 2000kg braked which is half a ton more than the car weighs,yet a wheezy 1.6 petrol mondeo can only tow 900kg braked.Maximum for unbraked is 750kg regardless of car.
Absolutely. 85% is not a rule, its a guideline. If you have an accident, the insurer will use what the car manufacturer states and has been approved to.