Hello Sharon,
The towing ratio is always calculated on the MTPLM of the trailer divided by the kerbweight of the car all multiplied by 100 to get to percent. This will always represent the worst case scenario. In practice when the car has any load in it the real ratio will improve, but the calculated value will always remain the same.
The maximum towed weight a car can manage is set by the manufacture. It is established by a series of tests so it is a real value not just a fixed proportion of any other weight. Consequently a cars max. towing limit could be less than 85% of its kerbweight, it all depends on the outcome of their tests.
The 85% figure comes from the caravan industry from long ago they never referred to it as a 'rule' only as guidance. It has no legal substance. However it does suggest that caravans should be kept as small as possible, but it is no guarantee the outfit will be safe or legal, however it is likely to improve the chances than if you go for a higher percentage.
The caravans nose weight is produced by the way you load it. You adjust it by rearranging the load inside the caravan. Here again the car manufacture will specify a maximum nose weight for the vehicle. The trailer manufacture will also specify a maximum nose weight. If there is a difference in the two values you must always observe the lowest value. I suggest you should aim to create a nose weight that settles in between 75 and 100% of the value by adjusting the position of the items in your caravan.
The weight towed by the car is not the whole weight of the trailer. Only the load on the trailers axle is towed, the nose weight is carried by the car. So for example if the total weight of a caravan is 1400Kg, and you arrange for a nose weight of 75Kg, then the towed weight is 1400-75 = 1325Kg.