Hello Sandra, and Richard,
Richard, your comments regarding the Opel Zafira may be fine and correct, but despite both the Open and Vauhall versions of the Zafira probably being built in the same factory from the same parts, you cannot assume that the models are specified the same way.
Prviously on this forum there have been questions about Zafiras, and for no obvious reason, the Opel Model was allowed a greater towing capacity than the Vauxhall versions. You would be in breech of the road regulations if you assumed a higher tow-load than the manufacture species for your car.
It is vitally important to ensure you get the information for your exact make,model & year of car/caravan, down to the engine version, trim and body style.
There have also been documented instances of where third party web sites and services have got the wrong information about a car or a caravan, and thus have produced inaccurate results. In other words do not trust them.
You should always get the relevant information about your specific model of car and caravan either from the owners information (V5 and hand book) or directly from the dealer or manufacture. The calculations required are not complicated.