Your tyre pressure can be calculated using the following formula:-
MTPLM ( Max tyre pressure / (Max tyre load x Number of wheels)) = Ideal tyre pressure
Example using typical values...
Max pressure = 44psi (Use the max pressure marked on the sidewalls of your tyres)
Max load = 487Kg (Use the max load marked on the sidewalls of your tyres)
Twin axle so 4 tyres (For a twin axle like yours use 4, works also for a single axle by using 2)
MTPLM = 1450Kg (Max weight permitted for your caravan)
1450(44/(487 x 4))=32.75psi
The answer comes in the same units used for the max tyre pressure, providing the units used for MTPLM and Max load are the same. I hope that makes sense.