In 40 years of driving, the rule of thumb I follow is to start at around 32-35 cold, and then adjust as appropriate. It is usually down, but I almost never wind up below 30.
What is an appropriate pressure in any particular circumstances, as a fraction of maximum pressure, depends in part on how much load the tire will see in relation to its maximum load rating.
As many others have said, the pressure on the tire sidewall is the maximum recommended for the tire by the tire manufacturer, and represents the maximum pressure to be used when the tire carries a load at or near its rated maximum.
If you are carrying a full capacity of passengers with their luggage, increasing pressure for that journey is a good idea.
An investment in a good tire gauge is wise.