The factory gauges often just had lines (and no numbers) for temperature graduations. Whether your car has instrumentation or a warning lamp, it is not very hard to calibrate it. This is easily done by putting a thermometer in a idling engine. I like the digital thermometers with a meat probe and you can even use them for cooking after calibrating your instrumentation. With the thermometer placed in the neck of the radiator, start the engine and let it idle. As the temperature gauge's needle passes a line on its face, record the temperature displayed by the thermometer. For those of you with cars having closed cooling systems with expansion/overflow tanks, you may need to syphon some coolant out to prevent the radiator from overflowing when hot. It would be wise to do a flush & fill with fresh antifreeze (ie, ethylene glycol) with a strength of 50% and not more than 68% (by volume).
When the flow increases substantially from the minimum observed from a cold engine, make a note of this temperature as it is the temperature at which the thermostat opens and the temperature gauge's reading. To increase the engine's temperature, block off the radiator with some cardboard, preferably the same size as the rad's cooling area to ensure minimal airflow through the rad. Be careful as the coolant will be very hot and you may need to siphon more coolant off to prevent the rad from overflowing. If your coolant has an insufficient ethylene glycol concentration, the coolant may start to boil.
With the front of the rad blocked off, the engine's temperature will continue to rise. Do not allow the engine to become hotter than the last graduation of the temperature gauge or when the temperature warning light comes on. Remove the cardboard from the from the rad and let the engine temperature stabilize at its normal operating temperature and then shut it down.