Hello again Mike,
You have asked bout including the driver in the towing ratio equation. For your information, the official towing ratio is calculated:-
MTPLM of the caravan / Kerbweight of the car. X 100%
It has to be said that this is a bit of a silly equation, because in practice most cars are actually heavier than their quoted kerbweight figure due to manufacturing tolerances and any OEM fitted extras on the car. Also the car will never be totally empty of passengers and luggage when it is towing, so the car will always be heavier than its Kbwt figure.
One might also think that the caravan will be less than its quoted MTPLM. I can't hold the same level of confidence about that. It is all too easy to use up all the caravans load capacity, and I am certain that some caravanners do actually overload their caravans so, but beware that is illegal.
The reason that the equation uses these two figurers is that they are consistent, and thus repeatable.
What you can legally tow is dependant on categories on your driving licence, and the maximum towed limit specified by the vehicle manufacturer. Each model of car is tested by the manufacture and they determine what the car can tow. Note this is not a guaranteed safe towing limit, no one can quote you a safe towing limit. The caravan industry suggest aiming at an 85% figure, but are unable or unwilling to explain why only 85%
I have no doubt that it is good sense to keep the weight of the trailer as small as possible, but I do not subscribe to suggested single figure of 85% which has no technical basis and no authority in law. Incidentally some cars are not even rated to tow 85% of their own weight, so it cannot be assumed to be anything more than guideline.
A good outfit is down to more than just a nominal weight ratio. The weight ratio is just one of several factors. The load distribution in the car and trailer are important to achieve the correct nose load, the condition of tyres and other mechanical bits. Road and weather conditions also play a major part in achieving a good tow.
But I think the biggest single factor that affects an outfit is how the driver chooses to drive.