Help please - tyre pressures 109"?

My LWB 1971 petrol 2.25 station wagon is fitted with Runway Enduro A/T 235/85 x 16 tyres.
At the moment I use 25psi all round, as advised by Haynes, but this is for the original equipment crossplys. Also my tyres are a bit fatter than standard. The vehicle is used as a family runabout, and also for pulling a heavy double horse trailer.
The steering is very heavy at parking speeds, so I'm tempted to raise the pressures, but I don't want to prejudice handling at higher speeds. Does anyone know where I can get advice on optimum pressures for my tyres ?

its a series, heavy steering is part of them.. what i do wiff tyres if am unsure of correct pressure is to inflate them till the 'belly' has just gone out of the sidewall. then keep a close eye on them fer uneven wear and adjust accordingly

My steerings very light on my 109, not at all heavy!