It's simple. Ignore everything except the max pressure on the tire sidewall.
Put the load on and look at the tires. The entire part of the tire that makes contact with the road (the tread profile) should be making contact. If it's bulging at the sidewall and the tread is only making contact at the edge, you are underinflated for the load. If the center of the tread profile is making contact, but the edges aren't, you have too much pressure.
Tire inflation is only a matter of getting the tire shape correct. Put as much air in as you need to get the shape correct, without exceeding the maximum pressure shown on the tire sidewall.
See these pictures for a visualization.
Tire inflation profile picturesIt's way, way easier than all those stickers on the vehicle confuse you (and a lot of repair shops) into thinking.
