If the Volvo has a maximum permissible towing limit of 1,800kg, then that's the limit of the weight of the caravan plus everything you put in it or on it (gas tanks, mover, battery, water cans , luggage, bikes etc).
As such, any caravan with an MTPLM of 1,800kg (the maximum weight the caravan can be loaded as specified by the manufacturer) is good to go. That's providing you don't overload it! However, you could choose a caravan that weighs 1,800kg with a MTPLM of 2,000kg and tow it with nothing in it and it would be OK, though I don't recommend it.
The maximum loaded weight of the caravan is the key in choosing your caravan, not the unladen weight of the caravan (kerb weight). The MIRO (mass in running order) is what the thing weighs unloaded. So don't choose your caravan based on what it weighs with nothing in it!
I don't agree with the 85% recommendation as it is based on the loaded weight of the car and the kerb weight of the car, both unloaded. NB: Not 85% of the maximum permissible towing limit of the car that I think you have understood. My advice is to go for the actual figures available from the sticker on your car and caravan manufacturer. The 85% recommendation is too arbitrary for my liking.
To be certain of the correct maximum towing limit of your Volvo, you have to look at the sticker:
Here you have the weight you can load the car up to (2,220kg) and the maximum gross train weight (4,020kg). Subtract one from the other gives me 1,800kg.
My caravan is a total pain because the sticker doesn't show the MIRO, so I've had to get it weighed so that I can work out how much stuff I can put in it (user payload) and not to exceed the MTPLN of 1,000kg.
