I have an 08 Mazda B4000. It comes equipped with P255/70R16's from the factory. The recommended tire pressure for them is 30psi. However, the previous owner had P245/75R16's installed. What's the best tire pressure for those tires? I know that between 30-35psi is typically recommended for a passenger car or light truck, but I just wanted to be sure.

Those tires are just modestly skinnier in width and taller in sidewall height than OEM, so I wold recommend simply following the Mazda's recommendation as printed on the driver's door label.

I would say you're very close. I recommend starting with 30-32 psi and watching for even wear. Have the alignment checked and reset if off too much.