proper tire inflation?

well i finally got the Michelin Cross Terrain 255/60/17 to replace the stock
Michelin LTX M/S 245/65/17 ... first impression is: they are great!

i feel like i got a pretty good deal on them at Tire Rack ($120 delivered)
and the dealer did the mount and install for $54.16 for all 4 tires ... i
could not believe that the dealer actually had the best deal on install ...

on the way home i checked the tire pressure and it ranged in the upper 30's
PSI in all 4 tires (not cold) ... has the dealer over-inflated the tires?

what is a good way of figuring out the approproriate inflation pressure for
this size tire on an infiniti QX4?