Tire rating is just the maximum pressure the tire structure can safely hold, and should always be at or above the vehicle's door plate recommended pressure.
For a given axle weight, the tire's contact patch will have the same surface area at a given pressure regardless of tire diameter and width, so for same width tires as stock just use the pressures on your door plate. This will give you a slightly shorter contact patch, less tread deflection, and longer tread wear. You can use a proportionately lower pressure if you've gone to a wider tire, giving you the same length of contact patch (so still slightly less tread deflection due to increased diameter) while the extra width gives you more surface area and lower ground loading pressure, which is better over fluid materials like pea gravel or dry sand.
You *may* use less pressure with a larger diameter, since it's the length of the contact patch that determines the amount the tread will deflect when going from round to flat as it rotates through the contact patch, and this tread deflection decreases as the tire diameter increases. It's the tread deflection that determines the heat loading in the rubber, which feeds into the tire's weight and speed ratings, as exceeding the heat loading is what makes a too-low tire overheat and become a blow out risk.
Be aware that the extra width will also lead to earlier hydroplaning over puddles, regardless of pressure, and choose your speed accordingly.
-dave