I must add a note Raywoods post.
Whilst I am sure all matching services genuinely want top offer good advice, All of them without exception have an achilles heel. The advice they can give is only as good as the information they have in their databases. We know from numerous posts on this and other forums, some of the information they have is wrong about some cars and caravans. and some models are not even listed.
It is essential you check the information they use is for your exact car and caravan, from make model year engine size fuel and output. body shape, and trim level. Do not accept anything that is not an exact match, as even similar models from the manufacturer may have important differences in relation to towing or being towed.
Ray has pointed out the caravan industries guidance about towing ratios and most of the matching services will adopt them and pronounce on that basis., It is important to understand that weight ratios alone do not determine if an outfit is a good match, but it is a usual starting point for when choosing a car or caravan. What is of probably more importance regarding the quality of the tow, is how you load the car and caravan, to get the best nose load, the proper maintenance of adjustment of brakes and tyres etc. And ultimately it how the outfit is driven. There are some very good towing advice courses offered by two main clubs and other various organisations.
The caravan industry tow ratio guidance figures should not be seen as fixed targets, and nothing about the outfits towing characteristics will have a step change if you exceed them, BUT it does make sense to always try to keep the weight of the caravan as small as possible. You must also be aware your car and caravan manufacturers specifications and towing limits must never be exceed under any circumstances even if the guidance suggests otherwise.