There does seem to be a whole heap of threads on here regarding tyre pressures and related confusion! Is there any kind of rule-of-thumb for it, or is it really a case of trial and error?

For me, I have Cooper Discoverer A/T3s on my 300TDI 110 - not by choice I hasten to add, they were on it when I bought it and one day I will change them to something which might give me back a smoother ride and a better turning circle(!). Meantime though, yes they do spend all their time on roads and no I have absolutely no clue what pressure to have in them! Can anyone help please?