I run light truck tyres on my rig (860kg).
I started with high pressure (55psi) but found the thing bounced all over the place at such a high pressure, so I started releasing pressure (5psi at a time) until I finally found a pressure which provided quite a good ride.
I found that to be about 40psi for me, but I'm sure it's different for each setup.
I'd suggest you try the same, but just be careful not to take the pressure too low, as that can lead to heat build up and a subsequent blowout.
Also, just beware of some tyres which come on Dunbier trailers. There were issues with some of them blowing out, and I think you'll find a post about it somewhere here if you do a quick search.