A lot of funny answers. Since the tire is supporting weight and a tire is just an air container with a specific volume of air capacity the pressure is always dependent on load.
Find out what the axle weight of the vehicle is first.
Using water and chalk and anything other than a good quality air gauge is just too funny. And I would never admit to "being in the business" and giving such unprofessional advice about pressures. I have to assume you were talking about a car with the correct, factory sized tire on it.
The pressure on the side of the tire is Not a recommendation. It is the maximum safe tire pressure for that particular tire.
Look up a load and inflation chart to find the load capacity of the tires you are thinking of using. Now look up the factory tire that came on that car and look up the load capacity of that tire. The tire you want to use should have at least the same capacity at the same pressure as what the car came with. Armed with the actual axle weight you can see what the safe limits are for the amount air pressure used.
