depends on tyre wall stiffness (ie using proxies and need a few psi more to hold the wall of paper mache up), type of roads used (motorway = higher pressure), ambient temperature, type of air in the tyre
tyre size is largely irrelevant as you need X ammount of tyre area on the ground to support the vehicle weight
Ive got the fronts on my vx down to about 22-23psi and the rears at 26psi as the walls are stiff, they are nitrogen filled and it behaves impecably. I think the astra with std 16" wheels is on about 29-30psi all round and drives fine.