The pressure marked on the sidewalls is a maximum, usually needed to reach the maximum load capacity of the tire (which is also marked on the sidewall) but not necessarily where the best handling is found with more-normal loads. That, as you surmised, varies from car to car and is found on a door panel or in the owner's manual or some other place specific to the car.
See for instance
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Racers and advanced sports car enthusiasts might "chalk" tires or use pyrometers to fine-tune the best tire pressure for what they are doing, or will bias it in a certain direction to get a specific cornering result; but regular folks doing ordinary driving, and who haven't made suspension mods or major changes in the size and type of tire, will probably find the best advice on the door panel.
If you're going to err, it's better to be a bit overinflated (within the sidewall rating) than grossly underinflated. But why err when you can be right? Remember to take the readings "cold" (i.e., before doing much driving) and to re-check every few weeks or before a long trip, whichever comes first -- tires lose air over time, and sometimes just one of them will do it faster.
Cheers,
--Joe