There are two methods of working out the correct pressure ( all right, three if you guess)
If you look at the markings on a tyre you can find the maximum load the tyre can support and the maximum pressure. From this you can work out the load that 1psi can support ( remember there are two wheels at each end of the car). If you know the front and rear weight of the car you can then find the pressure needed to carry the load of each end.
The second is a bit more impirical. I have seen the if you take the original tyre size and pressure you add 1.5psi for every inch in increase in diameter.
I've never checked to see if these give the same results as I use method three and add a bit to allow for the number of pot holes around here so they don't dent the rims (again). This does give you a harder ride but you have to live with that in a Furrari.