i guess you need to know the exact weight on the axle to work out what the pressure should be, i wonder is tyre manufactures do load/pressure graphs?
i always run mine at the pressure the tyre load carrying is rated at. my thinking behind this there is less tyre flex hence less heating, therefore less chance of a blowout. i think its 36psi for my tyres.
I am usually not as green as i am cabbage looking.